Podcast USB v0.3.0

SAFETY THIRD: reproduced and fixed the old FFmpeg 9.0.2 FLAC concatenation
failure for 179 and 180 (blocksize 1024 > 925). Convert non-MP3 parts separately
to compatible MP3, then join by copying compressed audio. Actual stage, source
formats and tool errors are now shown if preparation fails.

WHAT CHANGED IN v0.3

- MP3 audio is NOT re-encoded. Updating tags keeps compressed audio intact.
  Other formats are converted. Safety Third main + extra becomes one MP3.
- Concurrent unthrottled downloads (default 3, configurable 1-6), while one
  audio task processes. Network speed still depends on the host and connection.
- Live speed, bytes, download ETA, current audio ETA, expandable task list,
  activity history, Cancel and Skip this episode.
- Cancel/Skip operate during downloads and audio processing. Final verified
  USB copying finishes as a transaction; Cancel is disabled during that step.
- Resumable partial downloads and completed local archives are retained.
- Separate Choose -> Review -> Prepare -> Ready pages. Prepare & update USB
  is at the BOTTOM of the review page.
- Tick new episodes to DOWNLOAD. Tick old USB files to DELETE. Old folders
  start expanded. Select all and Unselect all are separate buttons.
- Feed descriptions, artwork, author, duration, season, episode, explicit flag,
  media type, download size and other supplied metadata are shown in Details.
- Rename output titles and edit artist/album BEFORE downloading.
- Artwork, descriptions and per-show Settings now live under Your lineup.
  Filter phrases, USB folder and per-show PC download override are there.
- Default PC download folder and parallel-download count are under Your lineup
  -> Download folders & processing settings. Each show can override the folder.
  Finished MP3s are stored there; raw sources live in .source-cache.
- Search shows artwork, description, creator, categories and episode count.
- Your podcasts is now a USB file editor: load files and edit filename, title,
  artist, album, track, date, genre and description. Audio stays intact.
- Existing settings are migrated automatically. Artwork/info are prefilled for
  the 12 existing shows and refreshed whenever their feeds are checked.

