feat: render TikTok photo posts into a video, and release 1.6.0

[vannt-dev]

Renders a TikTok photo post's images and music into one MP4 on the device, and
releases it as 1.6.0.

What this adds

• A slideshow option on TikTok photo posts, rendered on Android into a
  1080x1920 H.264 MP4 at three seconds an image, with the post's music
  transcoded to AAC and muxed alongside it.
• The option carries no download URL — there is nothing to fetch — so it is
  split off ahead of the download queue and rendered instead. The finished file
  enters history as a file that already exists.
• Progress reporting and cancellation for a render, since the encode runs
  outside the download queue and neither the progress bar nor the Cancel button
  could reach it.
• The option is shown only where it can actually run. A Dart conditional import
  cannot tell Android from iOS — both have dart.library.io — so a runtime
  Platform.isAndroid check in the renderer is what narrows support, and iOS,
  desktop and Web see no slideshow row at all.

Design notes worth reviewing

• The audio stage runs before the muxer is started. MediaMuxer refuses
  both MP3 in an MP4 and any track added after start(), so the whole track is
  transcoded and buffered first. A track that will not decode therefore cannot
  damage the picture: the render carries on and produces a silent video with a
  note on it.
• A rendered task's downloadUrl is the post URL, not empty. An empty one
  is the shape that 404s if anything ever fetches it.
• Retry re-extracts. The source a render was built from is held in memory
  while a failed task is persisted, so a retry after a restart recovers the
  source from the post rather than falling through to the download path.

Verification

tool/check_all.ps1 passes all 7 steps on this branch: 261 unit tests, 20
Android integration tests on an emulator, release Web build, Node tests,
formatting and analysis.

No pre-existing test was edited — git diff against the previous main for
test/ and integration_test/ is additions only, zero deletions.

Release

The final commit bumps to 1.6.0 with matching changelog notes, so merging this
starts the release workflow. The workflow's own three gates were checked
locally first: the changelog section is present and non-empty, v1.6.0 does
not exist, and the version increases over 1.5.2.

Merge with --rebase to keep main linear.