feat: next game date banner + demo mission "Blodprøve"

Add amber banner at top of Mission Tab showing next session date
(Lørdag 28. juni 2026). Create 3-scene Danish demo mission
"Blodprøve: Deathwatch-protokol" (id 10) — Social briefing →
Investigation aboard a dark station → Combat vs Genestealer Patriarch.

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

src/components/MissionTab.jsx

@@ -788,6 +788,15 @@ export default function MissionTab({ authedPlayer }) {
           </button>
         </div>
 
+        {/* Next game date banner */}
+        <div className="flex items-center gap-3 rounded-lg border border-amber-500/30 bg-amber-950/20 px-4 py-3">
+          <span className="text-lg">⚔️</span>
+          <div>
+            <span className="text-xs uppercase tracking-wide text-amber-400">Næste spildag</span>
+            <p className="font-semibold text-amber-100">Lørdag 28. juni 2026 — Deathwatch session</p>
+          </div>
+        </div>
+
         {error && <div className="rounded border border-red-500/40 bg-red-950/40 p-3 text-sm text-red-200">{error}</div>}
         {message && <div className="rounded border border-emerald-500/30 bg-emerald-950/30 p-3 text-sm text-emerald-200">{message}</div>}