Fix Rust backend: secrets to keychain, status recovery, shutdown, dedup

- Move git_token and Bedrock credentials to OS keychain instead of
  storing in plaintext projects.json via skip_serializing + keyring
- Fix project status stuck in Starting on container creation failure
  by resetting to Stopped on any error path
- Add granular store methods to reduce TOCTOU race window
- Add auth_mode, project path, and bedrock config change detection
  to container_needs_recreation with label-based fingerprinting
- Fix mutex held across async Docker API call in exec resize by
  cloning exec_id under lock then releasing before API call
- Add graceful shutdown via on_window_event to clean up exec sessions
- Extract compute_env_fingerprint and merge_claude_instructions helpers
  to eliminate code duplication in container.rs
- Remove unused thiserror dependency
- Return error instead of falling back to CWD when data dir unavailable

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/Cargo.toml

@@ -24,7 +24,6 @@ tokio = { version = "1", features = ["full"] }
 futures-util = "0.3"
 uuid = { version = "1", features = ["v4"] }
 chrono = { version = "0.4", features = ["serde"] }
-thiserror = "2"
 dirs = "6"
 log = "0.4"
 env_logger = "0.11"