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/config/definitions/defaults-core.ts

@@ -102,6 +102,7 @@ export const CORE_DEFAULT_CONFIG: Pick<
     openControllerSelect: 'Alt+C',
     openControllerDebug: 'Alt+Shift+C',
     toggleSubtitleSidebar: 'Backslash',
+    toggleNotificationHistory: 'Ctrl+N',
   },
   secondarySub: {
     secondarySubLanguages: [],