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/lib/generated/api.ts

@@ -5083,6 +5083,8 @@ export interface operations {
                 dir?: string;
                 /** @description Tag operator: AND (default) or OR */
                 tagOp?: string;
+                /** @description Restrict to undated documents (meta_date IS NULL) */
+                undated?: boolean;
                 /** @description Page number (0-indexed) */
                 page?: number;
                 /** @description Page size (max 100) */
@@ -5184,6 +5186,7 @@ export interface operations {
                 tagQ?: string;
                 status?: "PLACEHOLDER" | "UPLOADED" | "TRANSCRIBED" | "REVIEWED" | "ARCHIVED";
                 tagOp?: string;
+                undated?: boolean;
             };
             header?: never;
             path?: never;