Update the game from inside it (#13)

"Version X is available" in the top bar is now a button that opens an
Update popup:
- Update and restart gets the download link from the server (the redirect
  is not followed, as the Gitea link needs no token), downloads the new game
  next to the running one with a progress bar, checks it (complete, and an
  MZ or ELF file), swaps it in and restarts. The quiz is saved first and
  continues. On Windows the running .exe is renamed to .old, put back if
  the swap fails, and deleted at the next start.
- Download in browser opens the .exe, the Linux package, or the release
  page on other platforms.
- From the editor, an unwritable folder or another platform, only the
  browser download is offered, with the reason.

The message now fits between the tabs and the buttons in every theme; in
Nord and Christmas it ran into Sync. The macOS export preset is removed,
and the unused download_client_* functions are replaced by
fetch_client_download_url.

Closes #13

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_scene_split.gd

@@ -7,7 +7,7 @@ extends MainWindowTestSuite
 const MAIN_WINDOW_CHILDREN: Array[String] = [
 	"Background", "TopBar", "Quiz", "PopupBackdrop", "SearchPopup", "SyncPopup", "QRPopup",
 	"ShortcutsPopup", "StatisticsView", "AboutPopup", "LogPopup", "SoundtrackStatsPopup",
-	"SettingsPopup",
+	"SettingsPopup", "UpdatePopup",
 ]
 
 ## The parts of the quiz and the scene each one comes from.
@@ -28,7 +28,7 @@ const LOCKED_BY_SYNC: Array[String] = [
 ]
 
 const TOP_BAR_CHILDREN: Array[String] = [
-	"TopPanelContainer", "Title", "DebugLabel", "UpdateLabel", "OfflineLabel", "Buttons",
+	"TopPanelContainer", "Title", "DebugLabel", "UpdateBox", "OfflineLabel", "Buttons",
 	"QuizTab", "StatisticsTab",
 ]
 ## The buttons in the top bar's row on the right, in order.