Add optional push-to-talk mode

[rsre]

Summary

Adds an opt-in push-to-talk mode:

type: custom:webrtc-camera
url: doorbell
media: video,audio,microphone
ptt: true

• renders a Hold to talk button below the video
• captures and transmits the microphone only while the button is held
• detaches and stops the microphone track while idle
• mutes received camera audio while talking to prevent delayed echo
• requests microphone access from the button gesture for iOS/WebKit compatibility
• grants permission and negotiates an audio sender on the first hold, then shows Microphone ready — hold again to talk
• displays HTTPS, permission, device, and capture errors on the card
• defaults PTT cards to WebRTC mode unless a mode is explicitly configured

The implementation is contained in webrtc-camera.js; the vendored video-rtc.js is unchanged.