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/SegmentationColumn.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[];
@@ -17,14 +12,6 @@ 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'));
-}
 </script>
 
 {#if docs.length > 0}
@@ -53,11 +40,19 @@ function formatDate(dateStr: string): string {
 					>
 						<span class="font-serif text-sm text-ink">{doc.title}</span>
 						{#if doc.documentDate}
-							<span class="mt-0.5 text-xs text-ink-3">{formatDate(doc.documentDate)}</span>
+							<span class="mt-0.5 text-xs text-ink-3"
+								>{formatMCDate(doc.documentDate, getLocale())}</span
+							>
 						{/if}
 					</a>
 				</li>
 			{/each}
 		</ul>
 	</div>
+{:else}
+	<div
+		class="flex min-h-[120px] flex-col items-center justify-center rounded-sm border border-dashed border-line bg-surface/50 p-6 text-center"
+	>
+		<p class="text-xs text-ink-3">{m.mission_control_segmentation_empty()}</p>
+	</div>
 {/if}