feat(document): explain auto-generated title under the edit title field (#726)

Adds the FR-TITLE-005 helper line under the title input in DescriptionSection, shown only
on the single-document edit form via a new showTitleHelp prop (off for the new-document and
bulk-edit forms). It is wired to the input with aria-describedby and uses text-ink-3 (WCAG AA
on bg-surface). New Paraglide key form_helper_title_autogenerated in de/en/es. Adds a
component test for the helper + aria wiring and an end-to-end pass: create an auto-titled doc,
edit its date, and see the title follow on the detail page.

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

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

@@ -55,3 +55,26 @@ describe('DescriptionSection — onMount seeding (Felix B1/B2 fix regression fen
 		expect(input.value).toBe('Parent Value');
 	});
 });
+
+describe('DescriptionSection — auto-generated title helper (FR-TITLE-005)', () => {
+	it('shows the helper and wires aria-describedby when showTitleHelp is set', async () => {
+		render(DescriptionSection, { showTitleHelp: true });
+		const help = document.querySelector('#title-help') as HTMLElement;
+		expect(help).not.toBeNull();
+		expect(help.textContent?.trim().length ?? 0).toBeGreaterThan(0);
+		const titleInput = document.querySelector('input#title') as HTMLInputElement;
+		expect(titleInput.getAttribute('aria-describedby')).toBe('title-help');
+	});
+
+	it('omits the helper by default (e.g. the new-document form)', async () => {
+		render(DescriptionSection, {});
+		expect(document.querySelector('#title-help')).toBeNull();
+		const titleInput = document.querySelector('input#title') as HTMLInputElement;
+		expect(titleInput.getAttribute('aria-describedby')).toBeNull();
+	});
+
+	it('omits the helper when the title field is hidden (bulk edit)', async () => {
+		render(DescriptionSection, { showTitleHelp: true, hideTitle: true });
+		expect(document.querySelector('#title-help')).toBeNull();
+	});
+});