Fix base64 data URI detection: use startsWith instead of comma check

The previous "," check could incorrectly split raw base64 strings.
Added debug logging for base64 decode troubleshooting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api_server.py

@@ -328,9 +328,10 @@ def digitize_chart(
     # Accept both PIL Image and base64 string
     if isinstance(image, str):
         # Strip data URI prefix if present (e.g. "data:image/png;base64,...")
-        if "," in image:
+        if image.startswith("data:"):
             image = image.split(",", 1)[1]
         img_bytes = base64.b64decode(image)
+        print(f"  Base64 decoded: {len(img_bytes)} bytes, first 4 bytes: {img_bytes[:4]}")
         pil_img = Image.open(io.BytesIO(img_bytes)).convert("RGB")
         img_rgb = np.array(pil_img)
     else: