refactor(#240): deduplicate formatDate, use generated types, always-visible strip

- Add formatMCDate() to $lib/utils/date.ts (locale-aware, medium format);
  remove duplicated inline formatDate() from all three column components
- Replace local TranscriptionQueueItemDTO/TranscriptionWeeklyStatsDTO type
  declarations with imports from $lib/generated/api across all four components
- Add dashed empty states to SegmentationColumn and TranscriptionColumn
  (ReadyColumn already had one)
- Remove outer {#if} from MissionControlStrip so the section is always
  visible — each column owns its own empty state

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

frontend/src/lib/utils/date.ts

@@ -19,6 +19,19 @@ export function formatDate(isoDate: string, format: 'short' | 'long' = 'long'):
 	}).format(date);
 }
 
+/**
+ * Format an ISO date string for medium-length display (e.g. "15. Jun. 1920").
+ * Uses T12:00:00 to avoid UTC timezone off-by-one.
+ * Pass an explicit BCP 47 locale tag to respect the app locale; defaults to 'de-DE'.
+ */
+export function formatMCDate(isoDate: string, locale: string = 'de-DE'): string {
+	return new Intl.DateTimeFormat(locale, {
+		day: 'numeric',
+		month: 'short',
+		year: 'numeric'
+	}).format(new Date(isoDate + 'T12:00:00'));
+}
+
 /**
  * Converts an ISO date string (YYYY-MM-DD) to German display format (DD.MM.YYYY).
  * Returns an empty string for invalid or empty input.