feat: add PWA frontend for CYD ThermoPro dashboard

Add a Flask backend + SQLite + HTML/CSS/JS PWA that:
- Polls the CYD /api/latest endpoint
- Stores readings, sessions, and freeform notes in SQLite
- Displays probe cards with target setpoints and preset options
- Renders temperature history with Chart.js
- Serves a PWA manifest and dark-themed dashboard

Also update cyd-bridge/README.md with verified bringup instructions.

frontend/README.md

@@ -0,0 +1,50 @@
+# ThermoPro CYD Frontend
+
+A Progressive Web App (PWA) that consumes the CYD ThermoPro Bridge API, stores session history in SQLite, and displays probe temperatures with charts, target setpoints, and freeform notes.
+
+## Architecture
+
+```
+LXC/container
+├── Python backend (FastAPI/Flask)
+│   ├── Polls CYD /api/latest periodically
+│   ├── Stores readings + sessions + notes in SQLite
+│   └── Serves the static PWA and API
+└── PWA (HTML/CSS/JS)
+    ├── Dashboard with probe cards
+    ├── Temperature history charts
+    ├── Target setpoint editor
+    └── Session notes
+```
+
+## Requirements
+
+- Python 3.10+
+- `flask`, `requests`, `gunicorn` (or `uvicorn` if FastAPI)
+- SQLite (stdlib)
+
+## Running locally
+
+```bash
+cd frontend
+python -m venv .venv
+source .venv/bin/activate  # Windows: .venv\Scripts\activate
+pip install -r requirements.txt
+python app.py
+```
+
+Then open `http://localhost:5000`.
+
+## Configuration
+
+The backend needs to know the CYD IP. Set the environment variable:
+
+```bash
+export CYD_BASE_URL=http://192.168.2.55
+```
+
+or edit the default in `config.py`.
+
+## Deployment
+
+Serve the app behind a reverse proxy (e.g. Caddy or Nginx) in an LXC. The backend will continue polling the CYD as long as it can reach the device.