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/Cargo.lock

@@ -2084,7 +2084,6 @@ dependencies = [
  "tauri-plugin-dialog",
  "tauri-plugin-opener",
  "tauri-plugin-store",
- "tokio",
  "uuid",
  "walkdir",
 ]