feat: add in-browser dial pad with Twilio Voice SDK

- New DialPad component on Patient Connection page: clickable keypad,
  call/hangup/mute buttons, duration timer, keyboard input support
- Backend: POST /api/twilio/voice-token issues Access Token for browser
  Device; POST /api/twilio/webhook/voice-browser is the TwiML webhook
  Twilio calls to bridge the browser to the patient's phone
- TwiML App SID field added to Twilio Settings (stored in templates JSON)
- README: one-time Twilio Console setup instructions for the dial pad

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

package-lock.json

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