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>

themes/README.md

@@ -88,6 +88,9 @@ inspector or `theme_type_variation` in code:
 | Colour | Used for |
 |---|---|
 | `backdrop` | The dimming behind an open popup |
+| `first_point` | The flash over a player's row on their first point (see `common/anim.gd`) |
+| `match_point` | The glow over a player's row while they are one point from winning |
+| `flash` | The flash over a song's row when it starts playing or is answered |
 
 **ScoreGraph**: the graph of the game on the Winner popup, drawn from code.