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.

.github/workflows/ci.yml

@@ -23,11 +23,11 @@ jobs:
 
       - uses: Swatinem/rust-cache@v2.9.1
 
-      # libmpv for pstr-player.
-      - name: Install libmpv
+      # libmpv for pstr-player, WebKitGTK for the CAPTCHA window.
+      - name: Install libmpv and WebKitGTK
         run: |
           sudo apt-get update
-          sudo apt-get install -y libmpv-dev
+          sudo apt-get install -y libmpv-dev libwebkit2gtk-4.1-dev
 
       - run: cargo fmt --all -- --check