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/README.md

@@ -64,3 +64,8 @@ server on 127.0.0.1:18088 and RTP 10000–10019/udp. The script sets LAN IP = th
 IP = 127.0.0.1 (so ICE offers 127.0.0.1), restarts telephony, then signs in as a new user, enables
 the softphone with a fake microphone, and checks two-way audio to a baresip phone (171) in both
 directions.
+
+`m5-t2.mjs` (after `m5-t1.mjs`, same user) starts two baresip phones (172, 173) through
+`phones.mjs` and checks the softphone features: the incoming call dialog, DTMF, hold (music on hold
+at the other end), call waiting, blind and attended transfer, and "Use here" moving the softphone to
+a second tab (`cdp.mjs` `newTab()`).