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/windows/proton-stream.wixl.wxs

@@ -149,6 +149,13 @@
             <File Id="MpvDll" Source="$(var.MpvDll)" Name="libmpv-2.dll" KeyPath="yes" />
           </Component>
 
+          <!-- Loads WebView2 for the CAPTCHA window Proton sometimes puts
+               in front of a sign-in. A -gnu build links it as a DLL; the
+               WebView2 runtime itself comes with Windows. -->
+          <Component Id="WebViewLoader" Guid="3e8a89cf-cf3e-4cc7-b1c6-9d272b3210c3" Win64="yes">
+            <File Id="WebViewLoader" Source="$(var.WebViewLoader)" Name="WebView2Loader.dll" KeyPath="yes" />
+          </Component>
+
           <Component Id="ReadMe" Guid="5bb65b44-f5df-423b-8dcc-a753d7aef565" Win64="yes">
             <File Id="ReadMe" Source="$(var.ReadMe)" Name="README.md" KeyPath="yes" />
           </Component>
@@ -162,6 +169,7 @@
       <ComponentRef Id="AppExe" />
       <ComponentRef Id="CliExe" />
       <ComponentRef Id="MpvDll" />
+      <ComponentRef Id="WebViewLoader" />
       <ComponentRef Id="ReadMe" />
     </Feature>