feat(ui): add TUI with streaming agent integration and theming

Add a new terminal UI crate (crates/app/ui) built with ratatui providing an
interactive chat interface with real-time LLM streaming and tool visualization.

Features:
- Chat panel with horizontal padding for improved readability
- Input box with cursor navigation and command history
- Status bar with session statistics and uniform background styling
- 7 theme presets: Tokyo Night (default), Dracula, Catppuccin, Nord,
  Synthwave, Rose Pine, and Midnight Ocean
- Theme switching via /theme <name> and /themes commands
- Streaming LLM responses that accumulate into single messages
- Real-time tool call visualization with success/error states
- Session tracking (messages, tokens, tool calls, duration)
- REPL commands: /help, /status, /cost, /checkpoint, /rewind, /clear, /exit

Integration:
- CLI automatically launches TUI mode when running interactively (no prompt)
- Falls back to legacy text REPL with --no-tui flag
- Uses existing agent loop with streaming support
- Supports all existing tools (read, write, edit, glob, grep, bash)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/app/cli/Cargo.toml

@@ -19,6 +19,8 @@ tools-slash = { path = "../../tools/slash" }
 config-agent = { package = "config-agent", path = "../../platform/config" }
 permissions = { path = "../../platform/permissions" }
 hooks = { path = "../../platform/hooks" }
+ui = { path = "../ui" }
+atty = "0.2"
 futures-util = "0.3.31"
 
 [dev-dependencies]