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/components/ReadyColumn.svelte

@@ -1,15 +1,10 @@
 <script lang="ts">
 import * as m from '$lib/paraglide/messages.js';
 import { getLocale } from '$lib/paraglide/runtime.js';
+import { formatMCDate } from '$lib/utils/date.js';
+import type { components } from '$lib/generated/api';
 
-type TranscriptionQueueItemDTO = {
-	id: string;
-	title: string;
-	documentDate?: string;
-	annotationCount: number;
-	textedBlockCount: number;
-	reviewedBlockCount: number;
-};
+type TranscriptionQueueItemDTO = components['schemas']['TranscriptionQueueItemDTO'];
 
 interface Props {
 	docs: TranscriptionQueueItemDTO[];
@@ -18,14 +13,6 @@ interface Props {
 
 let { docs, weeklyCount }: Props = $props();
 
-function formatDate(dateStr: string): string {
-	return new Intl.DateTimeFormat(getLocale(), {
-		day: 'numeric',
-		month: 'short',
-		year: 'numeric'
-	}).format(new Date(dateStr + 'T12:00:00'));
-}
-
 function reviewedPct(doc: TranscriptionQueueItemDTO): number {
 	if (doc.annotationCount === 0) return 0;
 	return Math.round((doc.reviewedBlockCount / doc.annotationCount) * 100);
@@ -61,7 +48,8 @@ function reviewedPct(doc: TranscriptionQueueItemDTO): number {
 						<span class="font-serif text-sm text-ink">{doc.title}</span>
 						<div class="mt-0.5 flex items-center gap-2">
 							{#if doc.documentDate}
-								<span class="text-xs text-ink-3">{formatDate(doc.documentDate)}</span>
+								<span class="text-xs text-ink-3">{formatMCDate(doc.documentDate, getLocale())}</span
+								>
 							{/if}
 							{#if doc.textedBlockCount > 0}
 								<span class="text-xs font-semibold text-ink">