feat(enrich): add metadata enrichment queue UI

Home page shows "Needs metadata" card when incomplete documents exist.
/enrich list shows all incomplete documents; /enrich/[id] provides a
split PDF-preview + compact form view with Skip / Save / Save & reviewed
actions that auto-advance through the queue.

New document page gets Save vs Save & reviewed split. Edit page gets
"Mark for review" secondary button to push a document back into the queue.

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

frontend/src/routes/page.svelte.spec.ts

@@ -23,6 +23,7 @@ const emptyData = {
 	canAnnotate: false,
 	filters: { q: '', from: '', to: '', senderId: '', receiverId: '', tags: [] },
 	documents: [],
+	incompleteCount: 0,
 	initialValues: { senderName: '', receiverName: '' },
 	error: null
 };