test(e2e): add training footer positive-case test and fix broken selectors

- createEmptyDocument now uploads a minimal PDF so the Transkribieren
  button is rendered (requires isPdf = true in DocumentTopBar)
- add 'Transcribe coach — with blocks' describe: seeds a block via API,
  waits for blocks to settle in read mode, switches to edit, confirms
  'Für Training vormerken' is visible
- fix dark-theme axe test: ThemeToggle uses aria-label 'dark mode',
  not the previous /Farbmodus|theme/ regex

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

frontend/e2e/helpers/upload-empty-document.ts

@@ -1,10 +1,30 @@
 import type { APIRequestContext } from '@playwright/test';
+import path from 'path';
+import { fileURLToPath } from 'url';
+import fs from 'fs';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const PDF_FIXTURE = path.resolve(__dirname, '../fixtures/minimal.pdf');
 
 export async function createEmptyDocument(request: APIRequestContext): Promise<string> {
-	const res = await request.post('/api/documents', {
+	const createRes = await request.post('/api/documents', {
 		multipart: { title: 'E2E Transcribe Coach Test' }
 	});
-	if (!res.ok()) throw new Error(`Create document failed: ${res.status()}`);
-	const doc = await res.json();
-	return doc.id as string;
+	if (!createRes.ok()) throw new Error(`Create document failed: ${createRes.status()}`);
+	const doc = await createRes.json();
+	const docId = doc.id as string;
+
+	const uploadRes = await request.put(`/api/documents/${docId}`, {
+		multipart: {
+			title: doc.title,
+			file: {
+				name: 'minimal.pdf',
+				mimeType: 'application/pdf',
+				buffer: fs.readFileSync(PDF_FIXTURE)
+			}
+		}
+	});
+	if (!uploadRes.ok()) throw new Error(`Upload PDF failed: ${uploadRes.status()}`);
+
+	return docId;
 }