refactor: convert library scan to Server Action

- Add src/app/actions/library.ts with scanLibrary
- Update ScanButton to use Server Action
- Remove POST from api/komga/libraries/[libraryId]/scan route

docs/server-actions-plan.md

@@ -11,6 +11,7 @@
 | `POST /api/komga/favorites` | `addToFavorites()` | ✅ Done |
 | `DELETE /api/komga/favorites` | `removeFromFavorites()` | ✅ Done |
 | `PUT /api/preferences` | `updatePreferences()` | ✅ Done |
+| `POST /api/komga/libraries/[libraryId]/scan` | `scanLibrary()` | ✅ Done |
 
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