fix(studio,registry): unbreak studio test on main + make vignette demo legible

Two surgical changes, both isolated to the catalog-previews flow:

1. `packages/studio/src/player/hooks/usePlaybackKeyboard.test.ts`
   PR #842 changed `seek()` to take `(time, { keepPlaying: true })` for the
   A/E shortcuts. The keyboard-layout tests added by #839 still asserted the
   single-arg form. Both landed on main without cross-checking, so `main`
   itself has been failing Test/Windows since. Update the two assertions
   to match the new signature. Same fix Miguel already authored on
   `feat/studio-preview-pasteboard-bg`.

2. `registry/components/vignette/demo.html`
   The original demo captured a frame where the vignette was at its
   weakest point — the effect was nearly invisible in the static preview
   used by docs. Reworked the demo so:
   - The backdrop is a layered "cinematic still" (warm key + teal rim +
     dark falloff) and includes a centered subject ("moon"), so the
     vignette has a focal point to frame.
   - Vignette starts soft (size 70%, alpha 0.35) and ramps to a dramatic
     cinematic vignette (size 26%, alpha 0.92) over 1.6s.
   - Peak intensity holds across t≈3.0s, which is exactly where the
     catalog script samples the thumbnail (`Math.min(3.0, duration*0.6)`
     with duration=5).
   - Breathing motion in t=3.4–5.2s gives the video loop visible life
     without disturbing the still frame.

packages/studio/src/player/hooks/usePlaybackKeyboard.test.ts

@@ -93,7 +93,7 @@ describe("usePlaybackKeyboard — keyboard layout independence (#834)", () => {
       dispatch(keydown({ code: "KeyA", key: "a" }));
     });
 
-    expect(spies.seek).toHaveBeenCalledWith(1.5);
+    expect(spies.seek).toHaveBeenCalledWith(1.5, { keepPlaying: true });
   });
 
   it("'Jump to in-point' fires on AZERTY (physical KeyQ produces e.key='a')", () => {
@@ -104,7 +104,7 @@ describe("usePlaybackKeyboard — keyboard layout independence (#834)", () => {
       dispatch(keydown({ code: "KeyQ", key: "a" }));
     });
 
-    expect(spies.seek).toHaveBeenCalledWith(2.5);
+    expect(spies.seek).toHaveBeenCalledWith(2.5, { keepPlaying: true });
   });
 
   it("AZERTY 'A' physical key (e.key='q') no longer triggers in-point seek", () => {