Add built-in theme support with various pre-defined themes

- Introduce multiple built-in themes (`default_dark`, `default_light`, `gruvbox`, `dracula`, `solarized`, `midnight-ocean`, `rose-pine`, `monokai`, `material-dark`, `material-light`).
- Implement theming system with customizable color schemes for all UI components in the TUI.
- Include documentation for themes in `themes/README.md`.
- Add fallback mechanisms for default themes in case of parsing errors.
- Support custom themes with overrides via configuration.

themes/README.md

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+# OWLEN Built-in Themes
+
+This directory contains the built-in themes that are embedded into the OWLEN binary.
+
+## Available Themes
+
+- **default_dark** - High-contrast dark theme (default)
+- **default_light** - Clean light theme
+- **gruvbox** - Popular retro color scheme with warm tones
+- **dracula** - Dark theme with vibrant purple and cyan colors
+- **solarized** - Precision colors for optimal readability
+- **midnight-ocean** - Deep blue oceanic theme
+- **rose-pine** - Soho vibes with muted pastels
+- **monokai** - Classic code editor theme
+- **material-dark** - Google's Material Design dark variant
+- **material-light** - Google's Material Design light variant
+
+## Theme File Format
+
+Each theme is defined in TOML format with the following structure:
+
+```toml
+name = "theme-name"
+
+# Text colors
+text = "#ffffff"              # Main text color
+placeholder = "#808080"       # Placeholder/muted text
+
+# Background colors
+background = "#000000"        # Main background
+command_bar_background = "#111111"
+status_background = "#111111"
+
+# Border colors
+focused_panel_border = "#ff00ff"    # Active panel border
+unfocused_panel_border = "#800080"  # Inactive panel border
+
+# Message role colors
+user_message_role = "#00ffff"       # User messages
+assistant_message_role = "#ffff00"  # Assistant messages
+thinking_panel_title = "#ff00ff"    # Thinking panel title
+
+# Mode indicator colors (status bar)
+mode_normal = "#00ffff"
+mode_editing = "#00ff00"
+mode_model_selection = "#ffff00"
+mode_provider_selection = "#00ffff"
+mode_help = "#ff00ff"
+mode_visual = "#ff0080"
+mode_command = "#ffff00"
+
+# Selection and cursor
+selection_bg = "#0000ff"      # Selection background
+selection_fg = "#ffffff"      # Selection foreground
+cursor = "#ff0080"            # Cursor color
+
+# Status colors
+error = "#ff0000"             # Error messages
+info = "#00ff00"              # Info/success messages
+```
+
+## Color Format
+
+Colors can be specified in two formats:
+
+1. **Hex RGB**: `#rrggbb` (e.g., `#ff0000` for red, `#ff8800` for orange)
+2. **Named colors** (case-insensitive):
+   - **Basic**: `black`, `red`, `green`, `yellow`, `blue`, `magenta`, `cyan`, `white`
+   - **Gray variants**: `gray`, `grey`, `darkgray`, `darkgrey`
+   - **Light variants**: `lightred`, `lightgreen`, `lightyellow`, `lightblue`, `lightmagenta`, `lightcyan`
+
+**Note**: For colors not in the named list (like orange, purple, brown), use hex RGB format.
+
+OWLEN will display an error message on startup if a custom theme has invalid colors.
+
+## Creating Custom Themes
+
+To create your own theme:
+
+1. Copy one of these files to `~/.config/owlen/themes/`
+2. Rename and modify the colors
+3. Set `theme = "your-theme-name"` in `~/.config/owlen/config.toml`
+4. Or use `:theme your-theme-name` in OWLEN to switch
+
+## Embedding in Binary
+
+These theme files are embedded into the OWLEN binary at compile time using Rust's `include_str!()` macro. This ensures they're always available, even if the files are deleted from disk.
+
+Custom themes placed in `~/.config/owlen/themes/` will override built-in themes with the same name.

themes/default_dark.toml

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+name = "default_dark"
+text = "white"
+background = "black"
+focused_panel_border = "lightmagenta"
+unfocused_panel_border = "#5f1487"
+user_message_role = "lightblue"
+assistant_message_role = "yellow"
+thinking_panel_title = "lightmagenta"
+command_bar_background = "black"
+status_background = "black"
+mode_normal = "lightblue"
+mode_editing = "lightgreen"
+mode_model_selection = "lightyellow"
+mode_provider_selection = "lightcyan"
+mode_help = "lightmagenta"
+mode_visual = "magenta"
+mode_command = "yellow"
+selection_bg = "lightblue"
+selection_fg = "black"
+cursor = "magenta"
+placeholder = "darkgray"
+error = "red"
+info = "lightgreen"

themes/default_light.toml

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+name = "default_light"
+text = "black"
+background = "white"
+focused_panel_border = "#4a90e2"
+unfocused_panel_border = "#dddddd"
+user_message_role = "#0055a4"
+assistant_message_role = "#8e44ad"
+thinking_panel_title = "#8e44ad"
+command_bar_background = "white"
+status_background = "white"
+mode_normal = "#0055a4"
+mode_editing = "#2e8b57"
+mode_model_selection = "#b58900"
+mode_provider_selection = "#008b8b"
+mode_help = "#8e44ad"
+mode_visual = "#8e44ad"
+mode_command = "#b58900"
+selection_bg = "#a4c8f0"
+selection_fg = "black"
+cursor = "#d95f02"
+placeholder = "gray"
+error = "#c0392b"
+info = "green"

themes/dracula.toml

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+name = "dracula"
+text = "#f8f8f2"
+background = "#282a36"
+focused_panel_border = "#ff79c6"
+unfocused_panel_border = "#44475a"
+user_message_role = "#8be9fd"
+assistant_message_role = "#ff79c6"
+thinking_panel_title = "#bd93f9"
+command_bar_background = "#44475a"
+status_background = "#44475a"
+mode_normal = "#8be9fd"
+mode_editing = "#50fa7b"
+mode_model_selection = "#f1fa8c"
+mode_provider_selection = "#8be9fd"
+mode_help = "#bd93f9"
+mode_visual = "#ff79c6"
+mode_command = "#f1fa8c"
+selection_bg = "#44475a"
+selection_fg = "#f8f8f2"
+cursor = "#ff79c6"
+placeholder = "#6272a4"
+error = "#ff5555"
+info = "#50fa7b"

themes/gruvbox.toml

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+name = "gruvbox"
+text = "#ebdbb2"
+background = "#282828"
+focused_panel_border = "#fe8019"
+unfocused_panel_border = "#7c6f64"
+user_message_role = "#b8bb26"
+assistant_message_role = "#83a598"
+thinking_panel_title = "#d3869b"
+command_bar_background = "#3c3836"
+status_background = "#3c3836"
+mode_normal = "#83a598"
+mode_editing = "#b8bb26"
+mode_model_selection = "#fabd2f"
+mode_provider_selection = "#8ec07c"
+mode_help = "#d3869b"
+mode_visual = "#fe8019"
+mode_command = "#fabd2f"
+selection_bg = "#504945"
+selection_fg = "#ebdbb2"
+cursor = "#fe8019"
+placeholder = "#665c54"
+error = "#fb4934"
+info = "#b8bb26"

themes/material-dark.toml

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+name = "material-dark"
+text = "#eeffff"
+background = "#263238"
+focused_panel_border = "#80cbc4"
+unfocused_panel_border = "#546e7a"
+user_message_role = "#82aaff"
+assistant_message_role = "#c792ea"
+thinking_panel_title = "#ffcb6b"
+command_bar_background = "#212b30"
+status_background = "#212b30"
+mode_normal = "#82aaff"
+mode_editing = "#c3e88d"
+mode_model_selection = "#ffcb6b"
+mode_provider_selection = "#80cbc4"
+mode_help = "#c792ea"
+mode_visual = "#f07178"
+mode_command = "#ffcb6b"
+selection_bg = "#546e7a"
+selection_fg = "#eeffff"
+cursor = "#ffcc00"
+placeholder = "#546e7a"
+error = "#f07178"
+info = "#c3e88d"

themes/material-light.toml

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+name = "material-light"
+text = "#212121"
+background = "#eceff1"
+focused_panel_border = "#009688"
+unfocused_panel_border = "#b0bec5"
+user_message_role = "#448aff"
+assistant_message_role = "#7c4dff"
+thinking_panel_title = "#f57c00"
+command_bar_background = "#ffffff"
+status_background = "#ffffff"
+mode_normal = "#448aff"
+mode_editing = "#388e3c"
+mode_model_selection = "#f57c00"
+mode_provider_selection = "#009688"
+mode_help = "#7c4dff"
+mode_visual = "#d32f2f"
+mode_command = "#f57c00"
+selection_bg = "#b0bec5"
+selection_fg = "#212121"
+cursor = "#c2185b"
+placeholder = "#90a4ae"
+error = "#d32f2f"
+info = "#388e3c"

themes/midnight-ocean.toml

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+name = "midnight-ocean"
+text = "#c0caf5"
+background = "#0d1117"
+focused_panel_border = "#58a6ff"
+unfocused_panel_border = "#30363d"
+user_message_role = "#79c0ff"
+assistant_message_role = "#89ddff"
+thinking_panel_title = "#9ece6a"
+command_bar_background = "#161b22"
+status_background = "#161b22"
+mode_normal = "#79c0ff"
+mode_editing = "#9ece6a"
+mode_model_selection = "#ffd43b"
+mode_provider_selection = "#89ddff"
+mode_help = "#ff739d"
+mode_visual = "#f68cf5"
+mode_command = "#ffd43b"
+selection_bg = "#388bfd"
+selection_fg = "#0d1117"
+cursor = "#f68cf5"
+placeholder = "#6e7681"
+error = "#f85149"
+info = "#9ece6a"

themes/monokai.toml

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+name = "monokai"
+text = "#f8f8f2"
+background = "#272822"
+focused_panel_border = "#f92672"
+unfocused_panel_border = "#75715e"
+user_message_role = "#66d9ef"
+assistant_message_role = "#ae81ff"
+thinking_panel_title = "#e6db74"
+command_bar_background = "#272822"
+status_background = "#272822"
+mode_normal = "#66d9ef"
+mode_editing = "#a6e22e"
+mode_model_selection = "#e6db74"
+mode_provider_selection = "#66d9ef"
+mode_help = "#ae81ff"
+mode_visual = "#f92672"
+mode_command = "#e6db74"
+selection_bg = "#75715e"
+selection_fg = "#f8f8f2"
+cursor = "#f92672"
+placeholder = "#75715e"
+error = "#f92672"
+info = "#a6e22e"

themes/rose-pine.toml

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+name = "rose-pine"
+text = "#e0def4"
+background = "#191724"
+focused_panel_border = "#eb6f92"
+unfocused_panel_border = "#26233a"
+user_message_role = "#31748f"
+assistant_message_role = "#9ccfd8"
+thinking_panel_title = "#c4a7e7"
+command_bar_background = "#26233a"
+status_background = "#26233a"
+mode_normal = "#9ccfd8"
+mode_editing = "#ebbcba"
+mode_model_selection = "#f6c177"
+mode_provider_selection = "#31748f"
+mode_help = "#c4a7e7"
+mode_visual = "#eb6f92"
+mode_command = "#f6c177"
+selection_bg = "#403d52"
+selection_fg = "#e0def4"
+cursor = "#eb6f92"
+placeholder = "#6e6a86"
+error = "#eb6f92"
+info = "#9ccfd8"

themes/solarized.toml

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+name = "solarized"
+text = "#839496"
+background = "#002b36"
+focused_panel_border = "#268bd2"
+unfocused_panel_border = "#073642"
+user_message_role = "#2aa198"
+assistant_message_role = "#cb4b16"
+thinking_panel_title = "#6c71c4"
+command_bar_background = "#073642"
+status_background = "#073642"
+mode_normal = "#268bd2"
+mode_editing = "#859900"
+mode_model_selection = "#b58900"
+mode_provider_selection = "#2aa198"
+mode_help = "#6c71c4"
+mode_visual = "#d33682"
+mode_command = "#b58900"
+selection_bg = "#073642"
+selection_fg = "#93a1a1"
+cursor = "#d33682"
+placeholder = "#586e75"
+error = "#dc322f"
+info = "#859900"