fix(#240): make Mission Control Strip dark-mode compatible

Replace all hardcoded Tailwind colours with semantic tokens:
- bg-white → bg-surface (outer strip container)
- text-gray-400 → text-ink-3 (dates, meta text, empty-state copy)
- text-green-800 / text-green-700 → text-ink / text-ink-2 (headings, pulse, reviewed %)
- bg-green-50 / border-green-200 → bg-accent-bg / border-line (skill pill, weekly pulse badge)
- bg-ink text-white → bg-primary text-primary-fg (CTA buttons; dark: mint bg + navy text)
- hover:text-white → hover:text-primary-fg (ghost CTA hover text)
- focus-visible:ring-brand-navy → focus-visible:ring-focus-ring (all doc links)

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

frontend/src/lib/components/MissionControlStrip.svelte

@@ -31,8 +31,8 @@ let { segmentationDocs, transcriptionDocs, readyDocs, weeklyStats }: Props = $pr
 </script>
 
 {#if segmentationDocs.length > 0 || transcriptionDocs.length > 0 || readyDocs.length > 0}
-	<section class="mt-4 rounded-sm border border-line bg-white p-6">
-		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-gray-400 uppercase">
+	<section class="mt-4 rounded-sm border border-line bg-surface p-6">
+		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-ink-3 uppercase">
 			{m.mission_control_heading()}
 		</h2>
 		<div class="grid grid-cols-1 gap-4 sm:grid-cols-3">