Phase 9: onboarding, mobile & accessibility

- First-run welcome on the Campaigns page: what-the-app-is blurb, three feature
  highlights, and one-click "Try the sample campaign" (seedSampleCampaign) plus
  "Start your first campaign".
- Map token palette starts collapsed on small screens so the map gets the width.
- (Reduced-motion is already honoured globally in styles/globals.css.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/features/world/map/MapEditor.tsx

@@ -53,7 +53,8 @@ export function MapEditor({ map, campaign }: { map: BattleMap; campaign: Campaig
   const [gmDraw, setGmDraw] = useState(true);
   const [dims, setDims] = useState({ cols: 0, rows: 0 });
   const [editToken, setEditToken] = useState<string | null>(null);
-  const [showPalette, setShowPalette] = useState(true);
+  // Collapsed by default on small screens so the map gets the width.
+  const [showPalette, setShowPalette] = useState(() => typeof window === 'undefined' || window.innerWidth >= 880);
 
   // transient interaction state
   const [overlay, setOverlay] = useState<Overlay>({});