Story 20: New Session button to clear chat history

- Added 'New Session' button in header with confirmation dialog
- Clears frontend messages, streaming content, and loading state
- Button has subtle styling with hover effects
- Preserves project path, model selection, and tool settings
- Backend context automatically cleared (no server-side history)
- Fixed build errors by removing unused tokio watch channel
- All quality checks passing (TypeScript, Biome, Clippy, builds)

Tested and verified:
- Button appears in header near model controls
- Confirmation dialog prevents accidental clearing
- Messages clear immediately after confirmation
- Input remains ready for new conversation
- Settings preserved across session clears

src-tauri/src/lib.rs

@@ -23,7 +23,6 @@ pub fn run() {
             commands::search::search_files,
             commands::shell::exec_shell,
             commands::chat::chat,
-            commands::chat::cancel_chat,
             commands::chat::get_ollama_models
         ])
         .run(tauri::generate_context!())