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/backgrounds/animAurora.ts

@@ -51,7 +51,7 @@ const dark: CSSProperties = {
   backgroundSize: '260% 240%, 300% 260%, 240% 280%, 220% 200%, 100% 100%, 100% 100%',
   backgroundPosition: '0% 30%, 100% 70%, 50% 0%, 20% 80%, 50% 50%, 50% 50%',
   backgroundRepeat: 'no-repeat',
-  animation: `${auroraFlow} 60s ease-in-out infinite`,
+  animation: `${auroraFlow} 60s steps(180) infinite`,
 };
 
 const light: CSSProperties = {
@@ -75,7 +75,7 @@ const light: CSSProperties = {
   backgroundSize: '260% 240%, 300% 260%, 240% 280%, 220% 200%, 100% 100%, 100% 100%',
   backgroundPosition: '0% 30%, 100% 70%, 50% 0%, 20% 80%, 50% 50%, 50% 50%',
   backgroundRepeat: 'no-repeat',
-  animation: `${auroraFlow} 60s ease-in-out infinite`,
+  animation: `${auroraFlow} 60s steps(180) infinite`,
 };
 
 export const animAurora: ChatBgVariants = { dark, light };