🎬 Add DashboardOverviewScene component for Dashboard Overview (Scene 3)

Implements the 12-second Dashboard Overview scene featuring:
- 3D perspective entrance animation (rotateX 30° → 0°, scale 0.85 → 1.0)
- DashboardSidebar with staggered navigation items
- Three StatsCounter cards (Meetups: 204, Users: 1247, Events: 89)
- SparklineCharts showing trend data for each metric
- Activity feed section with recent meetup activities
- Quick stats section with country and user metrics
- Audio: card-slide.mp3 and ui-appear.mp3 sound effects
- Vignette overlay and dark theme styling

Includes 23 comprehensive tests covering all components and animations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

videos/src/PortalPresentation.tsx

@@ -2,6 +2,7 @@ import { AbsoluteFill, Sequence, useVideoConfig, Img, staticFile } from "remotio
 import { inconsolataFont } from "./fonts/inconsolata";
 import { PortalIntroScene } from "./scenes/portal/PortalIntroScene";
 import { PortalTitleScene } from "./scenes/portal/PortalTitleScene";
+import { DashboardOverviewScene } from "./scenes/portal/DashboardOverviewScene";
 
 /**
  * PortalPresentation - Main composition for the Einundzwanzig Portal presentation video
@@ -144,7 +145,7 @@ export const PortalPresentation: React.FC = () => {
         durationInFrames={sceneFrames.dashboardOverview.duration}
         premountFor={fps}
       >
-        <PlaceholderScene name="Dashboard Overview" sceneNumber={3} />
+        <DashboardOverviewScene />
       </Sequence>
 
       {/* Scene 4: Meine Meetups (12s) */}