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>

main/top_bar.gd

@@ -16,10 +16,19 @@ signal about_pressed
 signal qr_pressed
 ## Emitted when a tab is chosen: [code]&"quiz"[/code] or [code]&"statistics"[/code].
 signal tab_selected(tab: StringName)
+## Emitted when "Version X is available" is clicked, with the newest version.
+signal update_pressed(latest_version: String)
+
+## Space between the message in the middle and the tabs and buttons around it.
+const MIDDLE_GAP: float = 16.0
+
+## The newest release the server knows of, once asked, or empty.
+var latest_version: String = ""
 
 @onready var sync_button: Button = %SyncButton
 @onready var debug_label: Label = $DebugLabel
-@onready var update_label: Label = $UpdateLabel
+@onready var update_box: HBoxContainer = $UpdateBox
+@onready var update_button: Button = %UpdateButton
 @onready var offline_label: Label = $OfflineLabel
 @onready var qr_button: Button = %QRButton
 @onready var quiz_tab: Button = %QuizTab
@@ -36,6 +45,11 @@ func _ready() -> void:
 	qr_button.pressed.connect(qr_pressed.emit)
 	quiz_tab.pressed.connect(tab_selected.emit.bind(&"quiz"))
 	statistics_tab.pressed.connect(tab_selected.emit.bind(&"statistics"))
+	update_button.pressed.connect(func() -> void: update_pressed.emit(latest_version))
+	# The buttons are as wide as the theme makes them, so the message fits between
+	statistics_tab.item_rect_changed.connect(_fit_middle)
+	sync_button.item_rect_changed.connect(_fit_middle)
+	_fit_middle()
 	update_offline_state()
 	check_for_update()
 
@@ -51,14 +65,14 @@ func update_offline_state() -> void:
 	if offline:
 		offline_label.text = "Offline: " + Settings.music_folder.display_name()
 		offline_label.tooltip_text = Settings.music_folder.path
-		update_label.visible = false
+		update_box.visible = false
 
 
 ## Asks the server for the newest MusicPlayer release and tells the user when it is newer
 ## than this one. Not asked while a music folder is used.
 func check_for_update() -> void:
 	if Settings.is_offline():
-		update_label.visible = false
+		update_box.visible = false
 		return
 	MusicServerClient.fetch_latest_client_version(show_latest_version)
 
@@ -68,10 +82,20 @@ func check_for_update() -> void:
 func show_latest_version(latest: Variant) -> void:
 	var newer: bool = latest is String && is_newer_version(latest, Settings.version) \
 			&& !Settings.is_offline()
-	update_label.visible = newer
+	update_box.visible = newer
 	if newer:
-		update_label.text = "Version %s is available" % latest
-		update_label.tooltip_text = "You have %s" % Settings.version
+		latest_version = latest
+		update_button.text = "Version %s is available" % latest
+		update_button.tooltip_text = "You have %s. Click to update." % Settings.version
+
+
+# Fits "Version X is available" and the offline label between the tabs and the buttons
+func _fit_middle() -> void:
+	var left: float = statistics_tab.position.x + statistics_tab.size.x + MIDDLE_GAP
+	var right: float = sync_button.global_position.x - global_position.x - MIDDLE_GAP
+	for middle: Control in [update_box, offline_label]:
+		middle.position.x = left
+		middle.size.x = maxf(right - left, 0.0)
 
 
 ## Whether version [param latest] is newer than [param current], comparing numbers such as