Add camera selector dropdown to doorbell page

Replaces the hardcoded CAMERA_NAME with a CAMERAS list. The page renders
a dropdown when more than one camera is configured and remembers the
choice in localStorage; switching tears down and reopens the WebRTC
connection.

https://claude.ai/code/session_013XZ1vmgk78k2PEQ5DmJhF3

pi/README.md

@@ -52,13 +52,19 @@ sudo journalctl -u doorbell -f        # live logs
 Add to home screen (Chrome menu -> Add to home screen) for an app-like
 experience.
 
-## Changing the camera
+## Choosing the camera
 
-`server.py` near the top:
-```js
-const CAMERA_NAME = "front_door";
+`server.py` near the top has the list of cameras the page offers:
+```python
+CAMERAS = ["front_door"]
 ```
 
+Each name must match a `go2rtc.streams` entry in
+`frigate_config/config.yml` and the camera must be `enabled: true` in
+Frigate. With more than one entry the page shows a dropdown above the
+talk button; the choice is remembered per-browser in `localStorage`. The
+first entry is the default for new visitors.
+
 After editing:
 ```bash
 sudo systemctl restart doorbell