feat(ui): double showcase cadence + filter bar matches TopNav frost

- Showcase reveal cadence 80ms -> 160ms (slower, more deliberate one-at-a-time
  cascade per operator). Bump showcase.spec timer advances to cover 60x160ms.
- Gallery filter bar now uses the EXACT gradiated-obsidian frost + blur as
  TopNav (was a flat rgba(...,0.55) block), so the two read as one continuous
  piece of chrome with images visibly scrolling under both; the nav's
  transparent bottom edge against the bar's opaque top leaves a faint seam that
  separates them at the very top of scroll.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/stores/showcase.js

@@ -24,7 +24,8 @@ import { preloadImage } from '../utils/preloadImage.js'
 // fetch + decode (a cold TABLESAMPLE is a separate, query-side concern);
 // everything after it is buffer-smoothed.
 const PAGE = 3
-const CADENCE_MS = 80          // floor between fully-loaded reveals
+const CADENCE_MS = 160         // floor between fully-loaded reveals (doubled
+                              // 2026-06-04 — slower, more deliberate cadence)
 const PRIME = 6                // items buffered before the drain starts
 const BUFFER_TARGET = 30       // producer tops the queue up to this
 const BUFFER_MIN = 12          // ...and refills once the queue dips below this