Fix Story 12: Claude API key storage now working

- Fixed silent API key save failure by switching from keyring to Tauri store
- Removed keyring dependency (didn't work in macOS dev mode for unsigned apps)
- Implemented reliable cross-platform storage using tauri-plugin-store
- Added pendingMessageRef to preserve user message during API key dialog flow
- Refactored sendMessage to accept optional message parameter for retry
- Removed all debug logging and test code
- Removed unused entitlements.plist and macOS config
- API key now persists correctly between sessions
- Auto-retry after saving key works properly

Story 12 complete and archived.

src-tauri/Cargo.toml

@@ -32,5 +32,4 @@ chrono = { version = "0.4.42", features = ["serde"] }
 async-trait = "0.1.89"
 tauri-plugin-store = "2.4.1"
 tokio = { version = "1", features = ["sync"] }
-keyring = "3.2"
 eventsource-stream = "0.2.3"