test(documents): expand documents/[id]/edit page coverage

Adds doc.title, originalFilename fallback, cancel link href,
form.error pass-through.

4 new tests covering ~6 branches.

Refs #496.

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

frontend/src/routes/documents/[id]/edit/page.svelte.test.ts

@@ -56,4 +56,39 @@ describe('documents/[id]/edit page', () => {
 		);
 		expect(deleteBtn).toBeDefined();
 	});
+
+	it('uses doc.title in the document title when set', async () => {
+		render(DocumentEditPage, { props: { data: baseData(), form: undefined } });
+
+		await new Promise((r) => setTimeout(r, 30));
+		expect(document.title).toContain('Brief an Helene');
+	});
+
+	it('falls back to originalFilename when title is empty', async () => {
+		render(DocumentEditPage, {
+			props: {
+				data: baseData({ document: { ...baseDoc, title: '', originalFilename: 'fallback.pdf' } }),
+				form: undefined
+			}
+		});
+
+		await new Promise((r) => setTimeout(r, 30));
+		expect(document.title).toContain('fallback.pdf');
+	});
+
+	it('renders the cancel link to the document detail page', async () => {
+		render(DocumentEditPage, { props: { data: baseData(), form: undefined } });
+
+		const link = document.querySelector('a[href="/documents/d1"]');
+		expect(link).not.toBeNull();
+	});
+
+	it('passes form.error to DocumentEditLayout when form is set', async () => {
+		render(DocumentEditPage, {
+			props: { data: baseData(), form: { error: 'Save failed' } }
+		});
+
+		// Layout shows the form error in a banner — at minimum, error text is in the DOM
+		expect(document.body.textContent).toContain('Save failed');
+	});
 });