fix: WebRTC call bugs, hardening, camera switch, and network resilience

Bug fixes:
- Fix RemoteVideoMonitor killing group monitor job when primary track switches
- Add mutex protection to CallManager.initiateCall() to prevent state races
- Fix ICE restart offer never being sent to remote peer (was immediately
  replaced by a second offer from onRenegotiationNeeded)
- Fix duplicate duration timer in PiP connected call UI
- Fix error snackbar dismiss button not clearing the error
- Make PeerSessionManager thread-safe with synchronized blocks (accessed
  from WebRTC native threads and coroutine dispatchers concurrently)
- Make CallManager event handlers private (only called from onSignalingEvent)

Improvements:
- Replace fragile ICE candidate regex parsing with kotlinx.serialization JSON
- Respect DND/silent mode: only ring in NORMAL mode, only vibrate in VIBRATE
- Signal camera-off to remote peer by removing video track sender (instead
  of sending frozen/black frame)
- Clear CallSessionBridge on AccountViewModel.onCleared() to prevent stale
  references on account switch
- Custom TURN servers now replace defaults (instead of appending) so
  credentials can be rotated without an app update

New features:
- Front/back camera switch button (visible when video is enabled)
- Network transition handling: ConnectivityManager.NetworkCallback triggers
  ICE restart on all peers when network changes (WiFi/cellular handoff)

https://claude.ai/code/session_01JHn7skAibTrkVqsoWutgYe

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nipACWebRtcCalls/events/CallIceCandidateEvent.kt

@@ -27,6 +27,10 @@ import com.vitorpamplona.quartz.nip01Core.signers.eventTemplate
 import com.vitorpamplona.quartz.nip01Core.tags.people.pTag
 import com.vitorpamplona.quartz.nip31Alts.alt
 import com.vitorpamplona.quartz.nipACWebRtcCalls.tags.callId
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.int
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
 
 @Immutable
 class CallIceCandidateEvent(
@@ -37,27 +41,31 @@ class CallIceCandidateEvent(
     content: String,
     sig: HexKey,
 ) : WebRTCEvent(id, pubKey, createdAt, KIND, tags, content, sig) {
+    private val parsedJson by lazy {
+        try {
+            Json.parseToJsonElement(content).jsonObject
+        } catch (_: Exception) {
+            null
+        }
+    }
+
     fun candidateJson() = content
 
-    fun candidateSdp(): String = CANDIDATE_REGEX.find(content)?.groupValues?.get(1) ?: ""
+    fun candidateSdp(): String = parsedJson?.get("candidate")?.jsonPrimitive?.content ?: ""
 
-    fun sdpMid(): String = SDP_MID_REGEX.find(content)?.groupValues?.get(1) ?: "0"
+    fun sdpMid(): String = parsedJson?.get("sdpMid")?.jsonPrimitive?.content ?: "0"
 
     fun sdpMLineIndex(): Int =
-        SDP_MLINE_INDEX_REGEX
-            .find(content)
-            ?.groupValues
-            ?.get(1)
-            ?.toIntOrNull() ?: 0
+        try {
+            parsedJson?.get("sdpMLineIndex")?.jsonPrimitive?.int ?: 0
+        } catch (_: Exception) {
+            0
+        }
 
     companion object {
         const val KIND = 25052
         const val ALT_DESCRIPTION = "WebRTC ICE candidate"
 
-        private val CANDIDATE_REGEX = """"candidate"\s*:\s*"([^"]*)"""".toRegex()
-        private val SDP_MID_REGEX = """"sdpMid"\s*:\s*"([^"]*)"""".toRegex()
-        private val SDP_MLINE_INDEX_REGEX = """"sdpMLineIndex"\s*:\s*(\d+)""".toRegex()
-
         fun serializeCandidate(
             sdp: String,
             sdpMid: String,