fix(flutter): update banner false positive + lock-screen control routing

Update banner showing on identical versions:
- pubspec.yaml was stuck at the placeholder 0.1.0+1, so
  PackageInfo.version returned "0.1.0" while the server reported the
  actual release tag (e.g. "2026.05.10.1"). Comparison correctly said
  "newer" → banner always showed.
- Bump pubspec to 2026.05.11.0+1 so the local default matches the
  release cadence even before CI overrides it.
- Update flutter.yml release step to pass --build-name="${TAG#v}" so
  every tagged APK reports the tag as its PackageInfo.version. Future
  releases stop drifting from pubspec.
- Rewrite isVersionNewer to do component-wise int comparison with
  zero-padding: pub_semver.Version.parse rejects 4-part date versions
  like "2026.05.10.1", at which point the old code fell back to
  string inequality and treated "2026.05.10" as newer than itself
  vs "2026.05.10.0". Drop the pub_semver import (no longer used).

Lock-screen play/pause not responding:
- PlaybackState only listed MediaAction.seek in systemActions, which
  on Android 13+ means tapping the lock-screen play/pause button
  doesn't route back to the AudioHandler. Add play, pause,
  skipToNext, skipToPrevious to the set.
- Add androidCompactActionIndices: [0, 1, 2] so the compact
  notification view explicitly maps the three buttons.

Album art being smaller than the lock-screen frame is upstream of
this commit — the cover-cache writes whatever pixel dimensions the
server returns. If the server's /api/albums/<id>/cover returns small
thumbnails for these albums, the lock screen renders them at that
size. Worth a separate look at the server cover-emit path.

flutter_client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: minstrel
 description: Minstrel mobile client
 publish_to: 'none'
-version: 0.1.0+1
+version: 2026.05.11.0+1
 
 environment:
   sdk: '>=3.5.0 <4.0.0'