feat: Google Drive downloads, ffmpeg resilience, per-task temp isolation

[AbdelazizBarhoumi]

Google Drive support:

• GoogleDriveUrl parses share/open/uc/usercontent links into typed DriveError failures (private, not found, quota, folder) localized via asUserFacing()
• GoogleDriveResolver pre-resolves Drive files to a direct download URL with an OkHttp confirm-form follow; resolution runs at both info-fetch and download time because confirm tokens expire quickly
• DownloadUtil rewrites the resolved URL onto VideoInfo so yt-dlp never sees the share page; original URL preserved for display

FFmpeg resilience:

• FfmpegUtil self-tests the bundled libffmpeg.so before every download that needs it and fails fast with an actionable message instead of yt-dlp's opaque 'device not found' output
• Troubleshooting page gains an FFmpeg status card with re-init
• FFmpeg/Aria2c init failures no longer abort app startup

Thumbnail temp race fix (intermittent ENOENT on .jpg):

• DownloaderV2 runs up to 3 concurrent tasks, all previously sharing Download/Seal/tmp via -P temp:; yt-dlp reuses files it finds there (existing_file), so same-named thumbnails/.part files were deleted or moved mid-run by sibling tasks
• Each task now gets its own temp dir (task_<hash(taskId)>), removed on success and kept on failure so retries can still resume
• Reproduced locally: shared temp loses cross-task thumbnails in 10/12 concurrent runs, isolated temp 0/6

Also: values-be video_count lint fix, release keystore ignored. 23 unit tests (FfmpegUtil, GoogleDriveResolver with local MiniHttpServer, GoogleDriveUrl) + lint 0 errors + release build green.

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