feat: Backend cancellation support for interrupting model responses

Merged from feature/interrupt-on-type branch.

Backend cancellation infrastructure:
- Added tokio watch channel to SessionState for cancellation signaling
- Implemented cancel_chat command
- Modified chat command to use tokio::select! for racing requests vs cancellation
- When cancelled, HTTP request to Ollama is dropped and returns early
- Added tokio dependency with sync feature

Story updates:
- Story 13: Updated to use Stop button pattern (industry standard)
- Story 18: Created placeholder for streaming responses
- Stories 15-17: Placeholders for future features

Frontend changes:
- Removed auto-interrupt on typing behavior (too confusing)
- Backend infrastructure ready for Stop button implementation

Note: Story 13 UI (Stop button) not yet implemented - backend ready

src-tauri/src/lib.rs

@@ -23,6 +23,7 @@ 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!())