feat: seasonal theme overlays + improved animated chat backgrounds

Adds 11 CSS-only seasonal overlays (Halloween, Christmas, New Year, Autumn,
April Fool's, Lunar New Year, Valentine's Day, St. Patrick's Day, Earth Day,
Deep Space, Retro Arcade) with date-based auto-detection and a manual override
dropdown in Settings → Appearance → Seasonal Theme. All themes respect
prefers-reduced-motion. SeasonalEffect mounts at z-index 9997 in App.tsx.

Also rewrites all 5 animated chat background keyframes for smoother, more
organic motion: Digital Rain gains a phosphor glow flicker; Star Drift now
loops each layer by exactly its own tile size (no more seam); Grid Pulse adds
an independent brightness oscillation at a prime period; Aurora Flow drives
all four gradient layers through distinct paths; Fireflies adds glow-pulse and
opacity-blink animations at prime periods for unsynchronised bioluminescence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/settings/general/General.tsx

@@ -341,6 +341,10 @@ function Appearance() {
     'mentionHighlightColor',
   );
   const [fontFamily, setFontFamily] = useSetting(settingsAtom, 'fontFamily');
+  const [seasonalThemeOverride, setSeasonalThemeOverride] = useSetting(
+    settingsAtom,
+    'seasonalThemeOverride',
+  );
 
   return (
     <Box direction="Column" gap="100">
@@ -408,6 +412,51 @@ function Appearance() {
         />
       </SequenceCard>
 
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Seasonal Theme"
+          description="Decorative overlays that activate automatically on holidays and events, or choose one manually."
+          after={
+            <select
+              value={seasonalThemeOverride ?? 'auto'}
+              onChange={(e) =>
+                setSeasonalThemeOverride(
+                  e.target.value as typeof seasonalThemeOverride,
+                )
+              }
+              style={{
+                background: 'var(--bg-surface-variant)',
+                color: 'inherit',
+                border: '1px solid var(--border-interactive)',
+                borderRadius: '6px',
+                padding: '4px 8px',
+                fontSize: '14px',
+                fontFamily: 'inherit',
+                cursor: 'pointer',
+              }}
+            >
+              <option value="auto">🗓 Auto (date-based)</option>
+              <option value="off">Off</option>
+              <optgroup label="Holidays">
+                <option value="newyear">🎆 New Year (Dec 31–Jan 2)</option>
+                <option value="lunar">🏮 Lunar New Year (Jan 22–Feb 5)</option>
+                <option value="valentines">💖 Valentine&apos;s Day (Feb 10–15)</option>
+                <option value="stpatricks">🍀 St. Patrick&apos;s Day (Mar 15–18)</option>
+                <option value="aprilfools">🃏 April Fool&apos;s Day (Apr 1)</option>
+                <option value="earthday">🌱 Earth Day (Apr 22)</option>
+                <option value="autumn">🍂 Autumn (Sep 21–Oct 31)</option>
+                <option value="halloween">🎃 Halloween (Oct 15–Nov 1)</option>
+                <option value="christmas">❄️ Christmas (Dec 10–Jan 2)</option>
+              </optgroup>
+              <optgroup label="Events">
+                <option value="arcade">👾 Retro Arcade Day (Sep 12)</option>
+                <option value="deepspace">🚀 Deep Space Week (Oct 4–10)</option>
+              </optgroup>
+            </select>
+          }
+        />
+      </SequenceCard>
+
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Show Profile on Every Message"