feat(notifications): add notification history panel and overlay UX fixes

- New toggleNotificationHistory (Ctrl+N) session-scoped history panel; slides in from same edge as notification stack
- Overlay error/recovery toast follows notifications.overlayPosition; stack and history side seeded at startup
- Cold managed background startup initializes tray and visible overlay shell before tokenization warmups finish
- Add Update button to overlay update-available notifications
- Fix Ctrl+S sentence-card flow: only Anki progress notification, no duplicate status toast
- Fix overlay notification close/actions clickability above subtitle bars on Linux
- Increase pause-until-ready default timeout from 15s to 30s

src/core/services/overlay-shortcut-handler.test.ts

@@ -32,6 +32,7 @@ function makeShortcuts(overrides: Partial<ConfiguredShortcuts> = {}): Configured
     openControllerSelect: null,
     openControllerDebug: null,
     toggleSubtitleSidebar: null,
+    toggleNotificationHistory: null,
     ...overrides,
   };
 }