feat(documents): thread undated filter through the search loader + i18n

Parses ?undated strictly (=== 'true', mirroring the tagOp clamp), forwards
it as undated || undefined so the absent case drops out of the query, and
returns the flag in page data for the control to reflect. Adds the
docs_filter_undated_only toggle label and the explanatory
docs_range_excludes_undated empty-state copy in de/en/es. The badge reuses
the existing date_precision_unknown ("Datum unbekannt") key from #677.

OpenAPI types hand-edited for the new undated query param on /search and
/ids — CI must run `npm run generate:api` to confirm parity with the spec.

Refs #668

frontend/src/routes/documents/+page.server.ts

@@ -46,6 +46,8 @@ export async function load({ url, fetch }) {
 		: 'desc';
 	const tagQ = url.searchParams.get('tagQ') || '';
 	const tagOp = url.searchParams.get('tagOp') === 'OR' ? 'OR' : 'AND';
+	// Narrow the accepted truthy surface to exactly "true" (mirrors the tagOp clamp).
+	const undated = url.searchParams.get('undated') === 'true';
 	const page = Math.max(0, Number(url.searchParams.get('page') ?? '0') || 0);
 
 	const api = createApiClient(fetch);
@@ -66,6 +68,7 @@ export async function load({ url, fetch }) {
 						tag: tags.length ? tags : undefined,
 						tagQ: tagQ && !tags.length ? tagQ : undefined,
 						tagOp: tagOp === 'OR' ? 'OR' : undefined,
+						undated: undated || undefined,
 						sort,
 						dir: dir || undefined,
 						page,
@@ -94,6 +97,7 @@ export async function load({ url, fetch }) {
 			dir,
 			tagQ,
 			tagOp,
+			undated,
 			error: 'Daten konnten nicht geladen werden.' as string | null
 		};
 	}
@@ -124,6 +128,7 @@ export async function load({ url, fetch }) {
 		dir,
 		tagQ,
 		tagOp,
+		undated,
 		error: errorMessage
 	};
 }