Fix 7 bugs: async proxies, mode default, JS key serialization, validation

- Use asyncio.to_thread for proxy endpoints to avoid blocking event loop
- Add mode to DEFAULTS so it doesn't silently insert 0
- Use JSON serialization for keys in project_dynamic.js (with comma fallback)
- Validate path exists in change_path, friendly error on duplicate rename
- Remove unused exp param from rename closure
- Use deepcopy for DEFAULTS consistently

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

utils.py

@@ -30,6 +30,7 @@ DEFAULTS = {
     "cfg": 1.5,
 
     # --- Settings ---
+    "mode": 0,
     "camera": "static",
     "flf": 0.0,