fix: modal actually processes image with rotation/crop and fixes zoom

Critical fixes:
1. Modal now applies rotation and crop to image, returns processed blob
2. Frontend sends processed image to backend (not original)
3. Zoom slider min/max now calculated based on image size
4. Initial zoom shows entire image in canvas
5. Zoom constraints prevent over-zooming or under-zooming

User experience improved:
- Sees full item at start (auto-fit zoom)
- Can adjust rotation/crop smoothly
- Adjusted image is what gets saved (not original)
- Zoom slider works correctly for any image size

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

frontend/components/AIOnboarding.tsx

@@ -60,13 +60,12 @@ export default function AIOnboarding({ onCancel, onComplete, categories, invento
     setShowImageAdjustment(true);
   };
 
-  const handleImageAdjustmentConfirm = async (adjustments: { rotation: number; cropBounds: { x: number; y: number; width: number; height: number } } | null) => {
+  const handleImageAdjustmentConfirm = async (adjustments: any) => {
     if (!pendingItemData) return;
 
     const { index } = pendingItemData;
 
     if (adjustments && extractedItems[index]) {
-      // Override AI-detected values with user adjustments
       const updatedItems = [...extractedItems];
       updatedItems[index] = {
         ...updatedItems[index],
@@ -82,11 +81,27 @@ export default function AIOnboarding({ onCancel, onComplete, categories, invento
           user_adjusted: true
         }
       };
-      setExtractedItems(updatedItems);
+
+      // If modal processed and returned image blob, use that instead of original
+      if (adjustments.imageBlob) {
+        // Convert blob to base64 for extracted_image_bytes
+        const reader = new FileReader();
+        reader.onload = () => {
+          const base64 = (reader.result as string).split(',')[1];
+          updatedItems[index].extractedImageBlob = adjustments.imageBlob;
+          updatedItems[index]._base64ImageData = base64;
+          setExtractedItems(updatedItems);
+          hookConfirmSingleItem(index);
+        };
+        reader.readAsDataURL(adjustments.imageBlob);
+      } else {
+        setExtractedItems(updatedItems);
+        hookConfirmSingleItem(index);
+      }
+    } else {
+      hookConfirmSingleItem(index);
     }
 
-    // Now call the hook's confirm with the updated data
-    hookConfirmSingleItem(index);
     setShowImageAdjustment(false);
     setAdjustingItemIndex(null);
     setPendingItemData(null);