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.

packaging/PKGBUILD

@@ -23,7 +23,7 @@ license=('MIT')
 # mpv carries libmpv.so plus its headers on Arch, so it is both the runtime and
 # the build dependency. libsecret is the Secret Service side of the keyring —
 # the share fragment and custom password never touch shares.json.
-depends=('mpv' 'libsecret' 'hicolor-icon-theme')
+depends=('mpv' 'libsecret' 'webkit2gtk-4.1' 'hicolor-icon-theme')
 makedepends=('cargo')
 optdepends=('gnome-keyring: credential storage via Secret Service'
             'kwallet: credential storage via Secret Service on KDE')