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/fakes/fake_http_transport.gd

@@ -31,6 +31,9 @@ var requested_bodies: Array[String] = []
 ## The body of each request as bytes, e.g. an uploaded picture, in the same order as
 ## [member requested_paths].
 var requested_raw_bodies: Array[PackedByteArray] = []
+## Whether each request followed redirects, in the same order as
+## [member requested_paths].
+var requested_follow_redirects: Array[bool] = []
 ## How many tokens have been asked for.
 var token_requests: int = 0
 ## The JSON body of the last token request.
@@ -67,6 +70,14 @@ func uninstall() -> void:
 	Settings.queue_ids = _previous_queue_ids
 
 
+## Answers requests to [param path] with a redirect to [param location], like the
+## server's download routes. Requests that follow redirects get an empty 200 instead, as
+## if they had followed it.
+func respond_redirect(path: String, location: String) -> void:
+	_responses[path] = {"code": 302, "body": PackedByteArray(), "location": location}
+	_successes_left.erase(path)
+
+
 ## Answers requests to [param path] with [param data] encoded as JSON.
 func respond_json(path: String, data: Variant, response_code: int = 200) -> void:
 	respond_bytes(path, JSON.stringify(data).to_utf8_buffer(), response_code)
@@ -114,6 +125,7 @@ func request(
 		on_complete: Callable,
 		headers: PackedStringArray = PackedStringArray(),
 		body: PackedByteArray = PackedByteArray(),
+		follow_redirects: bool = true,
 ) -> void:
 	var path: String = url.trim_prefix(Settings.default_path + MusicServerClient.API_PATH)
 	if path == "/token" && method == HTTPClient.METHOD_POST:
@@ -129,17 +141,18 @@ func request(
 	requested_headers.append(headers)
 	requested_bodies.append(body.get_string_from_utf8())
 	requested_raw_bodies.append(body)
+	requested_follow_redirects.append(follow_redirects)
 	if !OPEN_PATHS.has(path.get_slice("?", 0)) && !_has_valid_token(headers):
 		on_complete.call(401, JSON.stringify({"error": "Unauthorized"}).to_utf8_buffer(),
 				PackedStringArray())
 		return
 	if _held_paths.has(path):
-		_held_paths[path].append(_answer.bind(path, on_complete))
+		_held_paths[path].append(_answer.bind(path, on_complete, follow_redirects))
 		return
-	_answer(path, on_complete)
+	_answer(path, on_complete, follow_redirects)
 
 
-func _answer(path: String, on_complete: Callable) -> void:
+func _answer(path: String, on_complete: Callable, follow_redirects: bool = true) -> void:
 	var headers: PackedStringArray = []
 	if !queue_id.is_empty() && path != "/token":
 		headers.append("X-Queue-Id: " + queue_id)
@@ -148,7 +161,13 @@ func _answer(path: String, on_complete: Callable) -> void:
 			on_complete.call(503, PackedByteArray(), headers)
 			return
 		_successes_left[path] -= 1
-	if _responses.has(path):
+	if _responses.has(path) && _responses[path].has("location"):
+		if follow_redirects:
+			on_complete.call(200, PackedByteArray(), headers)
+		else:
+			headers.append("Location: " + _responses[path].location)
+			on_complete.call(302, PackedByteArray(), headers)
+	elif _responses.has(path):
 		on_complete.call(_responses[path].code, _responses[path].body, headers)
 	else:
 		on_complete.call(404, PackedByteArray(), headers)