fix: dynamic video duration across ProductionForm + LivePreviewCanvas

- Expanded useVideoDurations to detect both form-sourced segments AND
  editable-slot video fields inside content scenes
- Wired videoDurations into ProductionForm (getTemplateDuration,
  compileExpressToTimeline, LivePreviewCanvas, ExportModal)
- LivePreviewCanvas now accepts videoDurations prop
- Simplified getTemplateDuration/compiler: any scene with a known
  video duration uses it, regardless of segmentSource type
- A 33s uploaded video now creates a 33s timeline, not 5s

src/utils/expressCompiler.ts

@@ -82,15 +82,15 @@ export function getAspectDimensions(aspect: string): { w: number; h: number } {
  * Compute total duration of template in seconds.
  * @param videoDurations Optional map of scene.id → actual video duration (seconds).
  *                       When provided, overrides the static durationSeconds for scenes
- *                       that source video from form uploads.
+ *                       that have user-uploaded video content.
  */
 export function getTemplateDuration(
   template: ExpressTemplate,
   videoDurations?: Record<string, number>,
 ): number {
   return template.scenes.reduce((sum, scene) => {
-    // If this scene has a form-sourced video and we know its actual duration, use it
-    if (videoDurations && scene.segmentSource === 'form' && videoDurations[scene.id]) {
+    // If we know the actual video duration for this scene, use it
+    if (videoDurations && videoDurations[scene.id]) {
       return sum + videoDurations[scene.id];
     }
     return sum + scene.durationSeconds;
@@ -121,8 +121,8 @@ export function compileExpressToTimeline(
 
   // Process each scene sequentially — the template's scenes are the sole source of truth
   for (const scene of template.scenes) {
-    // Use actual video duration if available for form-sourced scenes
-    const sceneDuration = (videoDurations && scene.segmentSource === 'form' && videoDurations[scene.id])
+    // Use actual video duration if available (from useVideoDurations)
+    const sceneDuration = (videoDurations && videoDurations[scene.id])
       ? videoDurations[scene.id]
       : scene.durationSeconds;
     const sceneDurFrames = sceneDuration * fps;