feat(frontend): render honest precision dates in detail, list and search

Wires formatDocumentDate/DocumentDate into the read sites: the document
detail top bar + metadata drawer (the drawer shows the visible "Originaltext:"
raw line for UNKNOWN/SEASON/APPROX), the search/list rows (DocumentRow,
mobile + desktop), and the document multi-select dropdown label. A MONTH or
SEASON document now reads "Juni 1916"/"Sommer 1916" everywhere instead of a
fabricated day.

Adds metaDatePrecision to the DocumentRow/DocumentMultiSelect test fixtures
(required on DocumentListItem since #671) and updates the multi-select label
assertion to the honest long date.

Refs #666

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/document/DocumentRow.svelte.test.ts

@@ -49,6 +49,7 @@ const baseItem = (overrides: Record<string, unknown> = {}) => ({
 	title: 'Brief 1923',
 	originalFilename: 'b.pdf',
 	documentDate: '1923-04-15',
+	metaDatePrecision: 'DAY' as const,
 	sender,
 	receivers: [receiver],
 	tags: [],