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/DocumentEditLayout.svelte

@@ -221,6 +221,7 @@ async function handleReplaceFile(e: Event) {
 					initialArchiveFolder={doc.archiveFolder ?? ''}
 					initialSummary={doc.summary ?? ''}
 					titleRequired={true}
+					showTitleHelp={true}
 				/>
 			</form>