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/test/showcase.spec.js

@@ -46,8 +46,8 @@ describe('showcase store: buffered cascade', () => {
     stubShowcase()
     const s = useShowcaseStore()
     const p = s.loadInitial()
-    // Past INITIAL_COUNT (60) × CADENCE (80ms) plus fetch microtasks.
-    await vi.advanceTimersByTimeAsync(6000)
+    // Past INITIAL_COUNT (60) × CADENCE (160ms = 9.6s) plus fetch microtasks.
+    await vi.advanceTimersByTimeAsync(12000)
     await p
     expect(s.images.length).toBe(60)
     const ids = s.images.map((i) => i.id)
@@ -65,7 +65,7 @@ describe('showcase store: buffered cascade', () => {
     // the cadence — a 1s window must not dump the whole buffer at once.
     expect(early).toBeGreaterThan(0)
     expect(early).toBeLessThan(60)
-    await vi.advanceTimersByTimeAsync(6000)
+    await vi.advanceTimersByTimeAsync(12000)
     await p
     expect(s.images.length).toBe(60)
   })
@@ -80,7 +80,7 @@ describe('showcase store: buffered cascade', () => {
     await vi.advanceTimersByTimeAsync(1000)
     expect(s.images.length).toBe(0)   // gated on the un-decoded first image
     release()
-    await vi.advanceTimersByTimeAsync(6000)
+    await vi.advanceTimersByTimeAsync(12000)
     await p
     expect(s.images.length).toBe(60)  // cascade proceeds once it decodes
   })