fix(TranscribeDragDemo): reactive prefersReducedMotion + bg-parchment token

- Replace one-shot $derived(.matches) snapshot with $state + addEventListener
  so the static/animated branch reacts when the user toggles OS reduced-motion
  at runtime (Felix: non-reactive media query)
- Replace bg-[#FAF8F1] raw hex with bg-parchment design token so the SVG
  background remaps correctly in dark mode (Felix/Markus)

Also update TranscriptionPanelHeader.svelte.test.ts to expect role="region"
after the HelpPopover ARIA fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionPanelHeader.svelte.test.ts

@@ -177,6 +177,6 @@ describe('TranscriptionPanelHeader', () => {
 
 		const helpBtn = document.querySelector('button[aria-expanded]') as HTMLButtonElement;
 		helpBtn.dispatchEvent(new MouseEvent('click', { bubbles: true }));
-		await vi.waitFor(() => expect(document.querySelector('[role="tooltip"]')).not.toBeNull());
+		await vi.waitFor(() => expect(document.querySelector('[role="region"]')).not.toBeNull());
 	});
 });