feat: add system tray with quick actions and minimize-to-tray

System tray:
- Always-visible tray icon with context menu
- Quick Scratchpad action from tray (triggers session creation
  via IPC bridge to renderer)
- Running session count in tray tooltip and menu
- Click tray icon to show/focus window
- Quit option in tray menu

Minimize to tray:
- New 'Minimize to tray on close' toggle in Settings > Appearance
- When enabled, closing the window hides to tray instead of
  quitting (configurable per-user, default off)
- macOS: hides dock icon when minimized to tray
- Proper force-quit handling (Cmd+Q on macOS, tray Quit)

Full IPC contract chain:
- minimizeToTray setting in WorkspaceSettings
- setMinimizeToTray channel + ElectronApi method
- Preload bridge, service handler, IPC registration
- Preserved through workspace normalization

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/shared/contracts/ipc.ts

@@ -195,6 +195,7 @@ export interface ElectronApi {
   setTheme(theme: AppearanceTheme): Promise<WorkspaceState>;
   setTerminalHeight(input: SetTerminalHeightInput): Promise<WorkspaceState>;
   setNotificationsEnabled(enabled: boolean): Promise<WorkspaceState>;
+  setMinimizeToTray(enabled: boolean): Promise<WorkspaceState>;
   describeTerminal(): Promise<TerminalSnapshot | undefined>;
   createTerminal(): Promise<TerminalSnapshot>;
   restartTerminal(): Promise<TerminalSnapshot>;
@@ -208,6 +209,7 @@ export interface ElectronApi {
   onTerminalExit(listener: (info: TerminalExitInfo) => void): () => void;
   onWorkspaceUpdated(listener: (workspace: WorkspaceState) => void): () => void;
   onSessionEvent(listener: (event: SessionEventRecord) => void): () => void;
+  onTrayCreateScratchpad(listener: () => void): () => void;
 }
 
 export interface RendererSelectionState {