fix: return 404 for missing assets instead of index.html fallback

Prevents MIME type mismatch when asset filenames change after rebuild.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server.py

@@ -551,6 +551,9 @@ class APIHandler(BaseHTTPRequestHandler):
             path = '/index.html'
         file_path = os.path.join(DIST_DIR, path.lstrip('/'))
         if not os.path.isfile(file_path):
+            # Only fallback to index.html for routes, not for asset files
+            if path.startswith('/assets/') or '.' in path.split('/')[-1]:
+                self._respond(404, {'error': 'Not found'}); return
             file_path = os.path.join(DIST_DIR, 'index.html')
         try:
             with open(file_path, 'rb') as f: