feat(clients): persist input settings, add mic input gain, fix iOS chat + VoiceOver

Input mode (VAD/PTT/Always-On), VAD threshold, and the new mic gain were
applied to the core + UI but never saved, so every relaunch reset to VAD
defaults. Each client now persists them and re-applies on connect:
  - iOS: UserDefaults (SessionState.loadAndApplyVoiceSettings + setter writes)
  - macOS: UserDefaults via MainWindowController didSet + loadPersistedAudioSettings
    (settings window also restores the VAD slider from the stored threshold)
  - Windows: new Models/VoiceSettings.cs (JSON at %AppData%\VoiceCat\voice.json,
    mirrors FeedbackSettings) loaded/applied in MainForm

Add global send-side mic gain API vc_set_input_gain (applied to MIC PCM in
on_capture_frame before the VAD gate, clamped to int16) + Swift/C# bindings,
and a 0-300% (default 100%) mic-volume slider on all three clients.

Fix iOS chat: ChatView called sendText(scope:.channel) with no targetId (0),
so channel messages went nowhere; now passes session.currentChannelId.

Fix iOS per-user tuning for VoiceOver: the tuning sheet was long-press
.contextMenu only (invisible to VoiceOver); UserRow now also exposes the same
buttons via .accessibilityActions (no visual change).

Verified: core builds clean; ctest 24/27 (3 pre-existing teardown crashes,
reproduced with changes stashed); VoiceCatMac + VoiceCatiOS (arm64 sim) build
SUCCEEDED; VoiceCat.Interop dotnet build succeeded. Windows App not built
(WinForms can't build on macOS) — follows existing patterns.

core/include/voicecat.h

@@ -386,6 +386,12 @@ VC_API vc_result vc_set_self_mute(vc_client* c, int mic_muted, int deafened);
  * 1.0 = unity (default), >1.0 amplifies. Always LOCAL — no protocol traffic. */
 VC_API vc_result vc_set_output_volume(vc_client* c, float gain);
 
+/* Send-side microphone input gain. Applied to captured MIC PCM before the VAD/PTT gate and
+ * Opus encode (so boosting a quiet mic also helps it cross the VAD threshold). gain 0.0 = silent,
+ * 1.0 = unity (default), >1.0 amplifies; the boosted signal is clamped to int16. MIC stream only;
+ * always LOCAL — no protocol traffic. */
+VC_API vc_result vc_set_input_gain(vc_client* c, float gain);
+
 /* Receive-side, per remote stream, all LOCAL (no protocol traffic) — docs/voice.md §10:
  * gain (0..) , mute, and listener-chosen noise reduction on a specific user's stream. */
 VC_API vc_result vc_set_remote_stream(vc_client* c, uint32_t user_id, uint32_t stream_id,