feat(seasonal): Halloween redesign — webs, a spider, bats, a ghost; no tint (#239)

Draft for human review (#239 must be approved and closed by a human).

- Removed the full-screen purple wash, fog and the blurred green "moon" that
  sat over the member list and made it unreadable.
- Corner cobwebs (generated radial threads + sagging rings), thin and
  readable in both themes, smaller on phones.
- A spider on a thread in the empty space under the member list (near the
  top on phones), bobbing and swaying.
- Every 30 s a group of four bats flaps across the top of the screen (a real
  bat silhouette; wing beat without flattening them into dashes).
- Every 50 s a small, see-through ghost rises and fades (outlined so it
  shows on light themes; text stays readable through it).
- Theme-aware ink via --hw-ink/--hw-web: dark silhouettes on light themes,
  pale dusk-violet on dark ones.
- Reduced motion: webs and the spider at rest. Settings swatch: night sky,
  crescent moon with a bat across it, a corner web.

Rendered and inspected: light/dark/phone at the bat and ghost moments,
reduced on desktop and phone, the Settings swatch; 60 fps on vs off.

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

src/app/components/seasonal/SeasonalEffect.tsx

@@ -59,7 +59,7 @@ function buildOverlayContent(
 ): React.ReactNode {
   switch (theme) {
     case 'halloween':
-      return <HalloweenOverlay reduced={reduced} />;
+      return <HalloweenOverlay reduced={reduced} preview={preview} />;
     case 'christmas':
       return <ChristmasOverlay reduced={reduced} />;
     case 'newyear':