fix(core,app,android,cli,packaging): harden account sign-in and watch-history sync

- Desktop shows Proton's CAPTCHA instead of failing the sign-in: a wry webview
  in a child process of the app (`pstr-app::verify`), confined to
  https://*.proton.me, hands the token back and the sign-in is retried. Linux
  packages and CI gain WebKitGTK 4.1; the Windows zip and MSI ship
  WebView2Loader.dll, without which a -gnu build does not start.
- The desktop keyring splits a secret too long for one entry across several,
  so a Proton session with many key salts survives Windows' 1280-unit cap.
  Short secrets stay one entry, so every existing secret still reads.
- `WatchSync` runs against a small `HistoryDrive` trait and is tested end to
  end against an in-memory Drive: two devices, newer wins, no re-upload of an
  unchanged history, a missing file written again, an unreadable own file
  never overwritten, a newer-format file left alone.
- An installation's file unwritten for 90 days is retired by the next device
  to sync, after its entries are merged and carried in that device's file.
- Removing a share keeps its watch state: sync would bring it back from the
  other devices anyway, and re-adding the share picks up where it left off.
- Watch history syncs once more on desktop exit (bounded to 3 s) and, on
  Android, through a WorkManager job enqueued when the app leaves the screen.
- The second-factor prompts say security keys are not supported.

android/app/src/main/java/io/narl/protonstream/MainActivity.kt

@@ -33,6 +33,7 @@ import io.narl.protonstream.playback.MpvPlaybackState
 import io.narl.protonstream.playback.NativeMpvHost
 import io.narl.protonstream.playback.PlaybackService
 import io.narl.protonstream.settings.SettingsStore
+import io.narl.protonstream.sync.WatchSyncWorker
 import io.narl.protonstream.ui.ProtonStreamApp
 import io.narl.protonstream.ui.theme.AppearanceState
 import io.narl.protonstream.ui.theme.ProtonStreamTheme
@@ -207,6 +208,9 @@ class MainActivity : ComponentActivity() {
 
     override fun onStop() {
         super.onStop()
+        // Where the viewer stopped is what their other devices want next, and
+        // a process in the background may not live to the next timed sync.
+        if (!isChangingConfigurations) WatchSyncWorker.enqueue(this)
         if (
             !isChangingConfigurations &&
             !isInPictureInPictureMode &&