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.spec.ts

@@ -22,6 +22,7 @@ function makeItem(overrides: Partial<DocumentListItem> = {}): DocumentListItem {
 		title: 'Testbrief',
 		originalFilename: 'testbrief.pdf',
 		documentDate: '2024-03-15',
+		metaDatePrecision: 'DAY',
 		sender: undefined,
 		receivers: [],
 		tags: [],