fix: run frontend without virtualenv

- Modify app.py to add vendor/ to sys.path so pip --target works.
- Add frontend/.gitignore to exclude vendor/, __pycache__, thermopro.db.
- Update README with pip --target instructions.

frontend/app.py

@@ -1,7 +1,14 @@
 import json
+import sys
 import threading
 import time
 from datetime import datetime, timezone
+from pathlib import Path
+
+# Add vendor/ to the import path so pip --target dependencies are found.
+vendor_path = Path(__file__).parent / "vendor"
+if vendor_path.exists():
+    sys.path.insert(0, str(vendor_path))
 
 import requests
 from flask import Flask, jsonify, request, render_template