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/types/runtime.ts

@@ -41,6 +41,7 @@ import type {
   RuntimeOptionState,
   RuntimeOptionValue,
 } from './runtime-options';
+import type { OverlayNotificationEventPayload } from './notification';
 
 export interface WindowGeometry {
   x: number;
@@ -405,6 +406,8 @@ export interface ElectronAPI {
   getOverlayLayer: () => 'visible' | 'modal' | null;
   onSubtitle: (callback: (data: SubtitleData) => void) => void;
   onOverlayPointerRecoveryRequested: (callback: () => void) => void;
+  onOverlayNotification: (callback: (payload: OverlayNotificationEventPayload) => void) => void;
+  sendOverlayNotificationAction?: (notificationId: string, actionId: string) => void;
   onVisibility: (callback: (visible: boolean) => void) => void;
   onSubtitlePosition: (callback: (position: SubtitlePosition | null) => void) => void;
   getOverlayVisibility: () => Promise<boolean>;