feat(seasonal): Autumn: leaves actually fall; real leaf shapes and palette (#237)

Draft for human review (#237 must be approved and closed by a human).

- Bug: every falling leaf sat in a leaf-sized wrapper pinned at the top of
  the screen with `contain: layout paint style`. Paint containment clips
  children to that box, so a leaf stopped being drawn as soon as it fell out
  of its first ~40 px: the animated theme showed almost nothing while the DOM
  reported 10–16 leaves on screen. Now `contain: layout style`.
- Leaf shapes: a flag-style maple (bold lobes, stem) that stays recognisable
  at 30 px (the old outline read as a star/splat), an oak with a stem, and an
  aspen; gradient body with darker veins.
- Palette: crimson, pumpkin orange, gold, russet, burnt orange (was muted
  tans that read as beige on the light theme).
- 12 leaves at 28–52 px, opacity 0.5–0.65 so text under a passing leaf stays
  readable; brightened on the dark themes, where they sank into the wash.
- Reduced motion: resting leaves moved into empty layout areas (middle of
  the left rail, under the member list), partly off-edge, instead of on the
  sidebar, members, composer and corner controls.

Rendered and inspected at 10/15/20 s in light, dark, phone and reduced
motion (screenshots on #237).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/seasonal/themes/Autumn.css.ts

@@ -1,4 +1,4 @@
-import { keyframes } from '@vanilla-extract/css';
+import { keyframes, style } from '@vanilla-extract/css';
 
 /**
  * Autumn overlay keyframes. Every animation touches ONLY `transform` and
@@ -89,3 +89,16 @@ export const animEmberPulse = keyframes({
   '50%': { opacity: '1' },
   '100%': { opacity: '0.82' },
 });
+
+/**
+ * [#237] Leaf blade. On the dark themes the crimson/russet leaves sank into the
+ * dark warm wash, so they are lifted there (brightness only, no layout/paint
+ * cost beyond the existing layer).
+ */
+export const leafBlade = style({
+  selectors: {
+    'body.dark-theme &, body.butter-theme &': {
+      filter: 'brightness(1.45) saturate(1.15) drop-shadow(0 1px 2px oklch(0.15 0.04 40 / 0.5))',
+    },
+  },
+});