softphone: dock panel, incoming dialog, hold/mute/DTMF, transfers, one tab

The dock opens a panel with a 12-key dialpad (typing, paste normalization,
Enter) and microphone/speaker selection. Call cards have mute, hold,
keypad (DTMF) and transfer: "Transfer now" (blind) or "Talk first"
(attended, with Complete/Cancel). An incoming call opens a dialog when
idle, or a card during a call (call waiting holds the first call). A
synthesized WebAudio ringtone and desktop notifications, auto-answer for
Call-Info answer-after=0, and a poor-connection warning from getStats.
One tab registers: a Web Lock plus a BroadcastChannel "Use here".

Found by e2e: Chrome can report canInsertDTMF=false, so DTMF falls back to
SIP INFO; after a REFER's 2xx NOTIFY the browser sends BYE itself (RFC
5589), otherwise SIP.js keeps the call up. apps/web/TESTING.md is the
12-step manual run for Chrome and Firefox (still open, roadmap note);
e2e m5-t2 automates most of it in headless Edge.

Task: M5-T2

scripts/e2e/m5-t1.mjs

@@ -165,6 +165,7 @@ try {
   await p.shot('m5-t1-01-ready');
 
   // Browser → desk phone.
+  await p.click('Keypad');
   await p.fill('Number to call', '171');
   await p.click('Call');
   await p.waitFor(`document.body.innerText.includes('On call')`, 'outgoing call answered', 15_000);