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/editor/EditorApp.tsx

@@ -623,6 +623,7 @@ export function EditorApp() {
                           previewAssetType={sc?.previewAssetType ?? null}
                           previewVideoAutostart={sc?.previewVideoAutostart ?? false}
                           previewRotationDeg={sc?.previewRotationDeg ?? 0}
+                          darkenScene={sc?.darkenScene ?? false}
                           previewBusy={previewBusy}
                           mediaAssets={sceneMediaAssets}
                           audioRefs={sceneAudioRefs}
@@ -632,6 +633,9 @@ export function EditorApp() {
                           onPreviewVideoAutostartChange={(next) =>
                             void actions.updateScene(sid, { previewVideoAutostart: next })
                           }
+                          onDarkenSceneChange={(next) =>
+                            void actions.updateScene(sid, { darkenScene: next })
+                          }
                           onTitleChange={(title) => void actions.updateScene(sid, { title })}
                           onDescriptionChange={(description) =>
                             void actions.updateScene(sid, { description })
@@ -1681,11 +1685,13 @@ type SceneInspectorProps = {
   previewAssetType: 'image' | 'video' | null;
   previewVideoAutostart: boolean;
   previewRotationDeg: 0 | 90 | 180 | 270;
+  darkenScene: boolean;
   previewBusy: boolean;
   mediaAssets: MediaAsset[];
   audioRefs: SceneAudioRef[];
   onAudioRefsChange: (next: SceneAudioRef[]) => void;
   onPreviewVideoAutostartChange: (next: boolean) => void;
+  onDarkenSceneChange: (next: boolean) => void;
   onTitleChange: (v: string) => void;
   onDescriptionChange: (v: string) => void;
   onImportPreview: () => void;
@@ -1788,11 +1794,13 @@ function SceneInspector({
   previewAssetType,
   previewVideoAutostart,
   previewRotationDeg,
+  darkenScene,
   previewBusy,
   mediaAssets,
   audioRefs,
   onAudioRefsChange,
   onPreviewVideoAutostartChange,
+  onDarkenSceneChange,
   onTitleChange,
   onDescriptionChange,
   onImportPreview,
@@ -1877,6 +1885,19 @@ function SceneInspector({
           </Button>
         ) : null}
       </div>
+      {previewAssetId && previewAssetType === 'image' ? (
+        <>
+          <div className={styles.spacer6} />
+          <label className={styles.checkboxLabel}>
+            <input
+              type="checkbox"
+              checked={darkenScene}
+              onChange={(e) => onDarkenSceneChange(e.target.checked)}
+            />
+            <span className={styles.spanSm}>{t('scene.darkenScene')}</span>
+          </label>
+        </>
+      ) : null}
       <div className={styles.spacer6} />
       <div className={styles.labelSm}>{t('scene.audio')}</div>
       <div className={styles.audioDrop}>