feat(notifications): add overlay notifications with position config

- Add Catppuccin Macchiato overlay notification stack with 3s transient timeout
- Add `notifications.overlayPosition` config (top-left | top | top-right)
- Route startup tokenization and subtitle annotation status through configured surfaces
- Deduplicate rapid subtitle mode toggle notifications
- Change `both` to mean overlay + system; add `osd-system` as legacy alias for old behavior
- Keep `osd`/`osd-system` as config-file-only legacy values; Settings UI offers overlay/system/both/none

src/shared/ipc/contracts.ts

@@ -41,6 +41,7 @@ export const IPC_CHANNELS = {
     reportOverlayContentBounds: 'overlay-content-bounds:report',
     reportOverlayInteractive: 'overlay-interactive:report',
     overlayModalOpened: 'overlay:modal-opened',
+    overlayNotificationAction: 'overlay:notification-action',
     toggleStatsOverlay: 'stats:toggle-overlay',
     markActiveVideoWatched: 'immersion:mark-active-video-watched',
     dispatchSessionAction: 'session-action:dispatch',
@@ -144,6 +145,7 @@ export const IPC_CHANNELS = {
     subtitleSidebarToggle: 'subtitle-sidebar:toggle',
     primarySubtitleBarToggle: 'primary-subtitle-bar:toggle',
     configHotReload: 'config:hot-reload',
+    overlayNotification: 'overlay:notification',
   },
 } as const;