feat(webrtc): end-to-end encrypted voice & video calls with adaptive codecs

Add 1:1 voice and video calling over the existing SAS-verified peer
connection. Audio and video tracks ride the same RTCPeerConnection as the
chat, bundled onto one DTLS-SRTP transport, so media inherits the session's
end-to-end encryption. SDP offer/answer is renegotiated in-band over the
verified data channel — no signalling server, so the media's DTLS
fingerprints are authenticated end-to-end. Calls are gated on a connected,
SAS-verified session.

Codecs & adaptation:
- Opus tuned for lossy links (in-band FEC, DTX, RED redundancy); audio is
  bandwidth-prioritised and never throttled.
- VP9/AV1 single-encoding SVC with H.264/VP8 fallback; video degrades by
  spatial/temporal layer.
- Runtime NetworkAdaptationController trims video bitrate on loss/RTT and
  recovers as the link clears — no renegotiation. Live connection-quality
  indicator (Excellent/Good/Fair/Weak) in the call UI.

In-call controls: mute, camera on/off (voice→video upgrade in-band),
camera flip, minimize-to-widget, hang up, and accept/decline for incoming
calls. Production logging disabled (DEBUG_MODE=false); temporary call
diagnostic logger removed. Codec rationale in docs/webrtc-config.md.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.4.10",
+  "version": "5.5.0",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {
@@ -11,7 +11,7 @@
     "dev": "npm run build && python -m http.server 8000",
     "watch": "npx tailwindcss -i src/styles/tw-input.css -o assets/tailwind.css --watch",
     "serve": "npx http-server -p 8000",
-    "test": "node tests/sas-verification.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs"
+    "test": "node tests/sas-verification.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs"
   },
   "keywords": [
     "p2p",