refactor(document): move ConversationThumbnail into lib/document

With the Briefwechsel view gone, lib/conversation/ held a single shared
component whose only consumer is lib/document/ThumbnailRow. Move it (and
its spec) into lib/document/, update the import, delete the now-empty
lib/conversation/ folder, and fix the stale frontend/CLAUDE.md lib map.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,112 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+
+import ConversationThumbnail from './ConversationThumbnail.svelte';
+
+afterEach(() => {
+	cleanup();
+});
+
+describe('ConversationThumbnail', () => {
+	it('renders the thumbnail image with a cache-busting v= query param', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: '1111',
+				thumbnailUrl: '/api/documents/1111/thumbnail?v=2026-04-10T09%3A00%3A00Z',
+				thumbnailAspect: 'PORTRAIT',
+				pageCount: 1
+			}
+		});
+
+		const img = document.querySelector('img') as HTMLImageElement | null;
+		expect(img).not.toBeNull();
+		expect(img!.getAttribute('src')).toContain('/api/documents/1111/thumbnail');
+		expect(img!.getAttribute('src')).toContain('v=');
+	});
+
+	it('uses portrait dimensions when aspect is PORTRAIT', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 'p1',
+				thumbnailUrl: '/api/documents/p1/thumbnail?v=2026-04-10T09%3A00%3A00Z',
+				thumbnailAspect: 'PORTRAIT',
+				pageCount: 1
+			}
+		});
+
+		const tile = document.querySelector('[data-testid="conv-thumb-tile"]') as HTMLElement;
+		expect(tile.getAttribute('data-aspect')).toBe('PORTRAIT');
+	});
+
+	it('uses landscape dimensions when aspect is LANDSCAPE', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 'l1',
+				thumbnailUrl: '/api/documents/l1/thumbnail?v=2026-04-10T09%3A00%3A00Z',
+				thumbnailAspect: 'LANDSCAPE',
+				pageCount: 1
+			}
+		});
+
+		const tile = document.querySelector('[data-testid="conv-thumb-tile"]') as HTMLElement;
+		expect(tile.getAttribute('data-aspect')).toBe('LANDSCAPE');
+	});
+
+	it('falls back to PORTRAIT when thumbnailAspect is missing', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 'n1',
+				thumbnailUrl: '/api/documents/n1/thumbnail?v=2026-04-10T09%3A00%3A00Z'
+			}
+		});
+
+		const tile = document.querySelector('[data-testid="conv-thumb-tile"]') as HTMLElement;
+		expect(tile.getAttribute('data-aspect')).toBe('PORTRAIT');
+	});
+
+	it('renders the page badge when pageCount is greater than 1', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 'm1',
+				thumbnailUrl: '/api/documents/m1/thumbnail?v=2026-04-10T09%3A00%3A00Z',
+				thumbnailAspect: 'PORTRAIT',
+				pageCount: 4
+			}
+		});
+
+		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', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 's1',
+				thumbnailUrl: '/api/documents/s1/thumbnail?v=2026-04-10T09%3A00%3A00Z',
+				thumbnailAspect: 'PORTRAIT',
+				pageCount: 1
+			}
+		});
+
+		const badge = document.querySelector('[data-testid="conv-thumb-page-badge"]');
+		expect(badge).toBeNull();
+	});
+
+	it('renders a skeleton placeholder when no thumbnailUrl is set yet', () => {
+		render(ConversationThumbnail, {
+			doc: {
+				id: 'blank',
+				thumbnailAspect: 'PORTRAIT'
+			}
+		});
+
+		expect(document.querySelector('img')).toBeNull();
+		const skeleton = document.querySelector('[data-testid="conv-thumb-skeleton"]');
+		expect(skeleton).not.toBeNull();
+		expect(skeleton!.className).toContain('motion-safe:animate-pulse');
+	});
+});