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/config/definitions/defaults-core.ts

@@ -15,6 +15,7 @@ export const CORE_DEFAULT_CONFIG: Pick<
   | 'subsync'
   | 'startupWarmups'
   | 'updates'
+  | 'notifications'
   | 'auto_start_overlay'
 > = {
   subtitlePosition: { yPercent: 10 },
@@ -129,5 +130,8 @@ export const CORE_DEFAULT_CONFIG: Pick<
     notificationType: 'system',
     channel: 'stable',
   },
+  notifications: {
+    overlayPosition: 'top-right',
+  },
   auto_start_overlay: true,
 };