Follow the GDScript style guide in every script

- Types on every parameter, return value and variable, and snake_case names
  (the player name limit is the constant MAX_NAME_LENGTH).
- Spelling fixes: *_triggered, character_selected, separator, caught_errors.
  The server's catched_errors key stays as the server sends it.
- Signals are connected with signal.connect() instead of by name.
- Code is ordered signals, constants, exported vars, vars, @onready vars,
  functions, with @onready on one line and two blank lines between functions.
- Every script starts with a ## description. Template comments, old LOC notes
  and commented-out code are gone, apart from the local play code.
- Lines are at most 100 characters, apart from the local play path.
- The screenshot tour turns off the blinking text cursor so pictures match.

tests/unit/test_music_server_client.gd

@@ -1,4 +1,5 @@
 extends GdUnitTestSuite
+## MusicServerClient: which paths it asks for and how it handles the answers.
 
 var _server: FakeHttpTransport
 
@@ -115,7 +116,8 @@ func test_mark_song_played_sends_put() -> void:
 
 
 func test_character_name_is_url_encoded() -> void:
-	MusicServerClient.fetch_character_image("Paper Mario.png", func(_body: PackedByteArray) -> void: pass)
+	MusicServerClient.fetch_character_image("Paper Mario.png",
+			func(_body: PackedByteArray) -> void: pass)
 
 	assert_array(_server.requested_paths).contains_exactly(["/character?name=Paper%20Mario.png"])