fix(ios): output muted with A2DP, session lifecycle, mic input issues

Three bugs causing no audio output and no mic input:

1. .voiceChat mode + A2DP = output muted. The .voiceChat mode uses hardware
   AEC/AGC/HPF but requires HFP-compatible routes. A2DP is NOT HFP — iOS
   mutes the output because it can't set up the voice processing pipeline on
   an A2DP route. Fix: use .default mode for Standard+A2DP (no hardware AEC,
   but audio routes correctly). .voiceChat kept for HFP and speaker modes.
   Added info warning in Settings UI for A2DP no-AEC.

2. Session lifecycle broken. stopMicStream() called deactivateAfterStreaming()
   which deactivated the AVAudioSession — but the AudioEngine keeps running for
   remote audio playback, so leaving voice killed all remote audio. And the
   session was never activated when a remote user started talking (only on
   Join Voice), so you couldn't hear anyone before joining voice. Fix:
   - ensureSessionActive() replaces activateForStreaming() — idempotent, called
     on Join Voice AND on .streamStarted (remote user starts talking).
   - stopMicStream() no longer deactivates the session.
   - deactivateSession() called only on disconnect from server.
   - isSessionActive flag tracks state, updated by interruption handler.

3. setPreferredInputNumberOfChannels(1) called for mono — unnecessary (1 is
   the default) and may put the session in a bad state on some devices. Fix:
   only call it when stereo is explicitly selected. Also handle empty input
   port ID (selecting 'Default' in the picker) correctly.

Added comprehensive route logging — after activation, logs the current output
and input route names so issues can be diagnosed from Console.app.

clients/apple/iOS/VoiceCatiOS/Views/SettingsView.swift

@@ -83,6 +83,16 @@ struct SettingsView: View {
                         .accessibilityLabel("Warning: Raw mode with speaker output may cause echo")
                     }
 
+                    if router.showsA2dpNoAecWarning {
+                        Label(
+                            "A2DP mode — no echo cancellation (hardware AEC unavailable)",
+                            systemImage: "info.circle.fill"
+                        )
+                        .foregroundStyle(.blue)
+                        .font(.caption)
+                        .accessibilityLabel("Info: A2DP output mode does not support hardware echo cancellation")
+                    }
+
                     // Capture channels: Mono vs Stereo
                     Picker("Channels", selection: Binding(
                         get: { router.captureChannels },