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/main/runtime/update/update-notifications.test.ts

@@ -1,8 +1,9 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { notifyUpdateAvailable } from './update-notifications';
+import type { OverlayNotificationPayload } from '../../../types/notification';
 
-test('notifyUpdateAvailable routes system and osd notifications from config', async () => {
+test('notifyUpdateAvailable routes notification surfaces from config', async () => {
   const calls: string[] = [];
   const deps = {
     showSystemNotification: (title: string, body: string) => {
@@ -11,19 +12,27 @@ test('notifyUpdateAvailable routes system and osd notifications from config', as
     showOsdNotification: async (message: string) => {
       calls.push(`osd:${message}`);
     },
+    showOverlayNotification: (payload: OverlayNotificationPayload) => {
+      calls.push(`overlay:${payload.title}:${payload.body ?? ''}`);
+    },
     log: (message: string) => {
       calls.push(`log:${message}`);
     },
   };
 
+  await notifyUpdateAvailable({ notificationType: 'overlay', version: '0.15.0' }, deps);
   await notifyUpdateAvailable({ notificationType: 'system', version: '0.15.0' }, deps);
   await notifyUpdateAvailable({ notificationType: 'both', version: '0.15.0' }, deps);
+  await notifyUpdateAvailable({ notificationType: 'osd-system', version: '0.15.0' }, deps);
   await notifyUpdateAvailable({ notificationType: 'none', version: '0.15.0' }, deps);
 
   assert.deepEqual(calls, [
+    'overlay:SubMiner update available:SubMiner v0.15.0 is available',
     'system:SubMiner update available:SubMiner v0.15.0 is available',
+    'overlay:SubMiner update available:SubMiner v0.15.0 is available',
     'system:SubMiner update available:SubMiner v0.15.0 is available',
     'osd:SubMiner v0.15.0 is available',
+    'system:SubMiner update available:SubMiner v0.15.0 is available',
   ]);
 });
 
@@ -39,6 +48,9 @@ test('notifyUpdateAvailable logs osd fallback when overlay notification fails',
       showOsdNotification: async () => {
         throw new Error('mpv disconnected');
       },
+      showOverlayNotification: () => {
+        calls.push('overlay');
+      },
       log: (message) => {
         calls.push(message);
       },
@@ -60,6 +72,9 @@ test('notifyUpdateAvailable logs non-error osd failures with thrown value', asyn
       showOsdNotification: async () => {
         throw 'mpv disconnected';
       },
+      showOverlayNotification: () => {
+        calls.push('overlay');
+      },
       log: (message) => {
         calls.push(message);
       },