perf(backgrounds): animated chat backgrounds cost ~5% CPU instead of 50 to 70% (#337)

background-position can't run on the compositor, so every frame of the
animated chat backgrounds repainted the whole chat area: 51 to 72% main-thread
busy while idle in a room at 60 fps. The drifts are slow (3 to 22 px/s,
under a pixel per frame), so each animation now uses steps(): about 12
updates/s for fireflies, stars and aurora and about 24/s for rain and
pulse.

Measured headless (20 s idle in a room, CDP TaskDuration):
  none 0.7% → 0.9%; rain 50.7 → 5.9; stars 71.2 → 4.5;
  pulse 64.9 → 5.8; aurora 58.9 → 4.2; fireflies 71.7 → 3.8.

Aurora and pulse used ease-in-out; with steps() their motion is now
even-paced. Pause animations / reduced motion still strip the animation.

Fixes #337

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/lotus/chatBackground.ts

@@ -149,6 +149,11 @@ export const getChatBg = (
   }
   // For animated backgrounds, promote the element to its own compositor layer so
   // background-position keyframes don't trigger repaints on descendant elements.
+  // [Gitea #337] background-position can't run on the compositor, so every
+  // animation frame repaints the whole chat area (45 to 70% main-thread busy
+  // at 60 fps). The drifts are slow (3–22 px/s), so each animation uses steps():
+  // ~12 updates/s for the slow ones, ~24/s for rain and pulse: visually the
+  // same, a fraction of the repaints.
   if (style.animation) {
     return { ...style, willChange: 'background-position', contain: 'paint' };
   }