Add animations to popups, points, songs, the winner and the tabs (#48)

Popups fade in while growing and close the reverse way, with the backdrop
fading along. The winner gets confetti, bouncing pictures and a score graph
that draws itself from the left. Filtered statistics cards and search rows
fade in, the tabs slide in from their side, and every button bounces on hover
and click (ButtonBounce, now added to all buttons).

In the player list the score pops on every point, the first point flashes the
row, and a player on match point glows with a "Match point!" tag. A song
starting fades in the title, flashes its row (kept across list rebuilds) and
pops the play button.

Common/anim.gd holds the helpers. A saved "Reduce animations" switch in
Settings turns all movement off; the tests use it. The flash colours are
theme colours (Palette first_point, match_point, flash) in every theme.

Closes #48

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

quiz/now_playing.gd

@@ -1,10 +1,16 @@
 class_name NowPlaying
 extends VBoxContainer
-## The soundtrack and song that are playing, in large text at the top of the quiz.
+## The soundtrack and song that are playing, in large text at the top of the quiz. They
+## fade in when they change.
 
 
 ## Shows [param soundtrack_title] and [param song_title]. The playlist hides them as
 ## question marks until the answer is shown.
 func show_song(soundtrack_title: String, song_title: String) -> void:
+	var changed: bool = $SoundtrackLabel.text != soundtrack_title \
+			|| $SongLabel.text != song_title
 	$SoundtrackLabel.text = soundtrack_title
 	$SongLabel.text = song_title
+	# A new song, or the answer being shown, fades in
+	if changed:
+		Anim.fade_in(self, Anim.SLOW)