feat(briefwechsel): ConversationThumbnail page badge legible at small sizes

Bumps the multi-page badge from text-xs (12px) / px-1.5 py-0.5 to
text-sm (14px) / px-2 py-1. Meets senior-legibility on a 320px phone
without crowding the 120-wide tile — the badge stays tucked in the
top-right corner.

Refs #305
Fixes @leonievoss senior-accessibility concern from PR review

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

frontend/src/lib/components/ConversationThumbnail.svelte.spec.ts

@@ -78,6 +78,9 @@ describe('ConversationThumbnail', () => {
 		const badge = document.querySelector('[data-testid="conv-thumb-page-badge"]') as HTMLElement;
 		expect(badge).not.toBeNull();
 		expect(badge.textContent).toContain('4');
+		// Senior-readable size: text-sm (14px) rather than text-xs (12px) on a
+		// small tile avoids marginal legibility on a 320px phone.
+		expect(badge.className).toContain('text-sm');
 	});
 
 	it('hides the page badge when pageCount is 1 or missing', () => {