Add Short songs and a choice of how random songs are picked (#30, #51)

Short songs (#30): a quiz settings switch and a Seconds per song row (30 by
default, 10-120 in steps of 5), saved with the quiz settings. Only part of
each quiz song plays, from a random point 10-60% into it, picked once per song
so restart and repeat play the same part. The progress bar, time and seeking
stay within the part, a hidden beginning counts inside it, and its end counts
as the song ending (repeat, stop or go to the next song as set).

Random songs (#51): the low-played switch in Settings is now a Normal / Low
played / Classic choice, stored as Settings.random_mode and not saved.
fetch_random_song(mode) picks /music/rand, /music/rand/low or
/music/rand/classic.

Closes #30
Closes #51

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

tests/unit/test_music_server_client.gd

@@ -59,16 +59,16 @@ func test_json_list_that_is_not_an_array_calls_on_failed() -> void:
 	assert_array(outcome).contains_exactly(["failed"])
 
 
-func test_fetch_random_song_uses_normal_endpoint() -> void:
-	MusicServerClient.fetch_random_song(false, func(_body: PackedByteArray) -> void: pass)
+@warning_ignore("unused_parameter")
+func test_fetch_random_song_uses_the_endpoint_of_its_mode(mode: Settings.RandomMode,
+		path: String, test_parameters: Array = [
+			[Settings.RandomMode.NORMAL, "/music/rand"],
+			[Settings.RandomMode.LOW_PLAYED, "/music/rand/low"],
+			[Settings.RandomMode.CLASSIC, "/music/rand/classic"],
+		]) -> void:
+	MusicServerClient.fetch_random_song(mode, func(_body: PackedByteArray) -> void: pass)
 
-	assert_array(_server.requested_paths).contains_exactly(["/music/rand"])
-
-
-func test_fetch_random_song_uses_low_played_endpoint() -> void:
-	MusicServerClient.fetch_random_song(true, func(_body: PackedByteArray) -> void: pass)
-
-	assert_array(_server.requested_paths).contains_exactly(["/music/rand/low"])
+	assert_array(_server.requested_paths).contains_exactly([path])
 
 
 func test_fetch_song_passes_raw_bytes() -> void:
@@ -103,7 +103,7 @@ func test_failed_request_calls_on_failed() -> void:
 	_server.respond_bytes("/music/rand", PackedByteArray(), 503)
 	var outcome: Array[String] = []
 
-	MusicServerClient.fetch_random_song(false,
+	MusicServerClient.fetch_random_song(Settings.RandomMode.NORMAL,
 			func(_body: PackedByteArray) -> void: outcome.append("done"),
 			func() -> void: outcome.append("failed"))
 
@@ -370,8 +370,6 @@ func test_delete_token_without_token_sends_nothing() -> void:
 func test_endpoint_paths_and_methods() -> void:
 	var noop: Callable = func(_data: Variant = null) -> void: pass
 	var calls: Array[Array] = [
-		[func() -> void: MusicServerClient.fetch_random_song_classic(noop),
-				"/music/rand/classic", HTTPClient.METHOD_GET],
 		[func() -> void: MusicServerClient.fetch_next_song(noop),
 				"/music/next", HTTPClient.METHOD_GET],
 		[func() -> void: MusicServerClient.fetch_previous_song(noop),