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/channels.ts

@@ -16,6 +16,7 @@ export const ipcChannels = {
   setTheme: 'settings:set-theme',
   setTerminalHeight: 'settings:set-terminal-height',
   setNotificationsEnabled: 'settings:set-notifications-enabled',
+  setMinimizeToTray: 'settings:set-minimize-to-tray',
   saveMcpServer: 'tooling:mcp:save',
   deleteMcpServer: 'tooling:mcp:delete',
   saveLspProfile: 'tooling:lsp:save',
@@ -54,4 +55,5 @@ export const ipcChannels = {
   workspaceUpdated: 'workspace:updated',
   sessionEvent: 'sessions:event',
   getQuota: 'sidecar:get-quota',
+  trayCreateScratchpad: 'tray:create-scratchpad',
 } as const;