feat(desktop): add close-to-tray option

[Cvv9]

Summary

Adds a Keep Buzz running when the window closes setting under General settings.
When enabled, closing the main window hides it instead of quitting. Windows and
Linux users can reopen or quit Buzz from a lazily-created system tray icon;
macOS continues to use the standard Dock reopen and application Quit behavior.

Why

Closing the only Buzz window currently exits the desktop process, which also
stops notifications and locally managed agents. Users who want Buzz to continue
working in the background need an explicit, persistent close-to-tray option
without changing the default quit path when native tray setup fails.

Implementation

• Persists the machine-local preference in localStorage and applies it to the
  Tauri backend on launch and whenever the toggle changes.
• Creates the Windows/Linux tray icon only while the feature is enabled.
• Fails closed: if tray creation fails, the preference is turned back off so
  closing the window still exits normally.
• Keeps the two process-wide close/quit lifecycle flags in the dedicated tray
  module rather than adding more community-agnostic state to AppState.
• Marks genuine application exits before teardown so the close handler never
  traps an explicit Quit or restart.

Validation

• cargo check --manifest-path desktop/src-tauri/Cargo.toml
• cargo test --manifest-path desktop/src-tauri/Cargo.toml tray::tests --lib
  — 4 passed
• pnpm -C desktop check:file-sizes
• Biome check for all six changed frontend files
• Rust formatting and git diff --check

The branch was refreshed onto current block/buzz main.