feat(windows): package for the Microsoft Store as an MSIX bundle

The Store's unpackaged EXE path would require Authenticode-signing the
installer and every PE file inside it. MSIX submissions are re-signed by the
Store instead, so this route needs no code-signing certificate. build-msix.ps1
mirrors build-installer.ps1 and consumes the same per-architecture build
artifacts, leaving the installer, portable archives and WinSparkle appcast
untouched.

One template generates the manifest for both architectures, carrying the
identity reserved in Partner Center. check_windows_msix.py recomputes the
package family name from the publisher DN, so a mistyped identity fails CI
rather than a submission, and it parses the script rather than running it
because root CI is Linux. Qualified logo assets are indexed into
resources.pri; without the altform-unplated variants the shell draws the
taskbar icon on an accent-coloured plate.

PlatformDetector.isPackagedInstall gates the in-app updater and the Liberapay
tile, which the read-only package directory and Store commerce policy
respectively rule out. Gating at runtime keeps one Windows build feeding both
the installer and the Store package.

lib/screens/settings/settings_screen.dart

@@ -203,7 +203,7 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab, Moun
 
                 _buildAdvancedSection(),
 
-                if (UpdateService.isUpdateCheckEnabled) ...[_buildUpdateSection()],
+                if (UpdateService.isUpdateCheckAvailable) ...[_buildUpdateSection()],
 
                 // Hidden on Android TV / tvOS (no document picker); desktop in
                 // force-TV mode keeps it — FilePickerService works there.