This commit introduces a new cloud music source: Google Drive. Users can now sign in with their Google account, add music folders from their Drive, and stream songs directly within the app.

- **Google Drive Integration**:
    - **Authentication**: Implements Google Sign-In using the Credential Manager API. The server-side auth code flow is used to obtain access and refresh tokens for the Google Drive API.
    - **API Service**: Adds `GDriveApiService`, a lightweight OkHttp client for interacting with the Google Drive REST API v3 (listing files/folders, creating folders, token management).
    - **Repository**: Introduces `GDriveRepository` to manage authentication state, API calls, and local database caching. It handles token persistence and refresh logic.
    - **Database**:
        - Adds `gdrive_songs` and `gdrive_folders` tables to the database with a new `MIGRATION_21_22`.
        - Adds `GDriveDao` for accessing GDrive-specific data.
        - Updates `MusicDao` to support clearing GDrive songs from the unified library.
    - **Unified Library Sync**: GDrive songs are parsed and integrated into the main music library, with stable IDs generated for songs, albums, and artists. Sync logic correctly handles additions and deletions.

- **Streaming & Security**:
    - **GDrive Stream Proxy**: Adds `GDriveStreamProxy`, a local Ktor-based server that handles `gdrive://` URIs. It proxies requests to the Drive API, attaching the necessary `Authorization` header and automatically refreshing expired tokens.
    - **Cloud Stream Security**:
        - Creates a shared `CloudStreamSecurity` object to centralize validation logic for all cloud streaming proxies (Google Drive, Netease, Telegram).
        - Implements robust validation for `Range` headers, content length, content types, and upstream URLs to prevent SSRF and other potential proxy abuse.
        - Hardens existing `NeteaseStreamProxy` and `TelegramStreamProxy` by applying the new security checks.

- **UI & UX**:
    - **Login Flow**: Adds a new `GDriveLoginActivity` with a guided flow for signing in, browsing Drive folders, creating a dedicated "PixelPlay Music" folder, and selecting a source folder.
    - **Dashboard**: Creates a `GDriveDashboardScreen` for managing synced folders, initiating manual syncs, and logging out.
    - Updates `Song` model and `SongEntity` to recognize `gdrive://` content URIs.

- **Dependencies**:
    - Adds Google Sign-In libraries (`androidx.credentials`, `credentials-play-services-auth`, `com.google.android.libraries.identity.googleid`).