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/ui/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "ui"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+color-eyre = "0.6"
+crossterm = { version = "0.28", features = ["event-stream"] }
+ratatui = "0.28"
+tokio = { version = "1", features = ["full"] }
+futures = "0.3"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+unicode-width = "0.2"
+textwrap = "0.16"
+
+# Internal dependencies
+agent-core = { path = "../../core/agent" }
+permissions = { path = "../../platform/permissions" }
+llm-ollama = { path = "../../llm/ollama" }
+config-agent = { path = "../../platform/config" }