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/components/shared/LivePreviewCanvas.tsx

@@ -36,6 +36,8 @@ export interface LivePreviewCanvasProps {
   statusLabel?: string;
   /** Whether all required fields are complete */
   isComplete?: boolean;
+  /** Video duration overrides per scene (from useVideoDurations) */
+  videoDurations?: Record<string, number>;
 }
 
 /** Format frame number to mm:ss */
@@ -66,6 +68,7 @@ export const LivePreviewCanvas: React.FC<LivePreviewCanvasProps> = ({
   playerRef: externalRef,
   statusLabel,
   isComplete = false,
+  videoDurations,
 }) => {
   const internalRef = useRef<BradlyPlayerRef>(null);
   const playerRef = externalRef || internalRef;
@@ -76,13 +79,13 @@ export const LivePreviewCanvas: React.FC<LivePreviewCanvasProps> = ({
   const isScrubbing = useRef(false);
 
   const fps = 30;
-  const totalDuration = getTemplateDuration(template);
+  const totalDuration = getTemplateDuration(template, videoDurations);
   const totalFrames = Math.max(30, totalDuration * fps);
   const dimensions = getAspectDimensions(template.aspectRatio);
 
   // Compile template to timeline (reactive to fieldData + mediaFits)
   const compiled = useMemo(() => {
-    const result = compileExpressToTimeline(template, fieldData, designMD, brand);
+    const result = compileExpressToTimeline(template, fieldData, designMD, brand, videoDurations);
     // Strip transitions and apply mediaFit overrides
     result.elements = result.elements.map(el => {
       const fieldId = el.sourceFieldId;
@@ -97,7 +100,7 @@ export const LivePreviewCanvas: React.FC<LivePreviewCanvasProps> = ({
       };
     });
     return result;
-  }, [template, fieldData, designMD, brand, mediaFits, containBgColors]);
+  }, [template, fieldData, designMD, brand, mediaFits, containBgColors, videoDurations]);
 
   const playerInputProps = useMemo(() => ({
     designMD,