feat(scenes): add scene darkness reveal and fix project reopen crash

Add darkenScene with Opening brush in presentation, persist reveal strokes per scene during a session, and close projects properly on return to home. Harden dnd asset streaming to avoid Windows main-process crashes when reopening projects.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/shared/PresentationView.tsx

@@ -4,7 +4,9 @@ import { computeTimeSec } from '../../main/video/videoPlaybackStore';
 import type { SessionState } from '../../shared/ipc/contracts';
 
 import { PixiEffectsOverlay } from './effects/PxiEffectsOverlay';
+import { SceneDarknessOverlay } from './effects/SceneDarknessOverlay';
 import { useEffectsState } from './effects/useEffectsState';
+import { useSceneDarknessState } from './effects/useSceneDarknessState';
 import styles from './PresentationView.module.css';
 import { RotatedImage } from './RotatedImage';
 import { useAssetUrl } from './useAssetImageUrl';
@@ -25,6 +27,7 @@ export function PresentationView({
   showEffects = true,
 }: PresentationViewProps) {
   const [fxState] = useEffectsState();
+  const [sdState] = useSceneDarknessState();
   const [vp] = useVideoPlaybackState();
   const videoElRef = useRef<HTMLVideoElement | null>(null);
   const [contentRect, setContentRect] = React.useState<{ x: number; y: number; w: number; h: number } | null>(
@@ -132,6 +135,9 @@ export function PresentationView({
           }
         />
       ) : null}
+      {showEffects && scene?.previewAssetType === 'image' && scene.darkenScene && contentRect ? (
+        <SceneDarknessOverlay state={sdState} overlayAlpha={1} viewport={contentRect} />
+      ) : null}
       {showTitle ? (
         <div className={styles.titleWrap}>
           <div className={compact ? styles.titleCompact : styles.titleFull}>