fix: improve file size handling and error messages

- Reduce server-side limit to 5MB for upload endpoint
- Increase client-side resizing threshold to 4MB
- Add specific error handling for 413 responses
- Add more detailed logging for file sizes
- Improve user error messages

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

api/src/rehearsalhub/routers/auth.py

@@ -91,13 +91,13 @@ async def upload_avatar(
             detail="Only image files are allowed"
         )
     
-    # Validate file size (10MB limit)
-    max_size = 10 * 1024 * 1024  # 10MB
+    # Validate file size (5MB limit for upload endpoint)
+    max_size = 5 * 1024 * 1024  # 5MB
     if file.size > max_size:
         print(f"File too large: {file.size} bytes (max {max_size})")
         raise HTTPException(
             status_code=status.HTTP_413_REQUEST_ENTITY_TOO_LARGE,
-            detail=f"File too large. Maximum size is {max_size / 1024 / 1024}MB"
+            detail=f"File too large. Maximum size is {max_size / 1024 / 1024}MB. Please resize your image and try again."
         )
     
     # Create uploads directory if it doesn't exist