M5: Windows client moderation UI; add C ABI getters for account list, user mute/deafen, channel topic

core/src/core/client.h

@@ -90,6 +90,7 @@ struct vc_client {
     vc_result reset_password(const char* username, const char* new_password);
     vc_result delete_account(const char* username);
     vc_result list_accounts();
+    vc_result get_account_list(vc_account_list* out);
     vc_result get_permissions(vc_permissions* out);
 
     vc_connection_state state() const {
@@ -145,6 +146,11 @@ struct vc_client {
     voicecat::session::SessionModel session_model_;
     mutable std::mutex              session_model_mu_;
 
+    // M5: last ListAccountsResult snapshot, populated on io_thread_ when
+    // VC_EVENT_ACCOUNT_LIST fires and read by vc_get_account_list on caller threads.
+    std::vector<voicecat::v1::AccountEntry> last_account_list_;
+    mutable std::mutex                      account_list_mu_;
+
     // ── M4: TOFU server-identity gate ───────────────────────────────────────────
     std::unique_ptr<voicecat::crypto::TofuStore> tofu_store_;  // owns the pin file
     std::mutex              tofu_mu_;