Kiosk: System Name storage, validation, sync, and sent on license validation

Adds local System Name persistence (trimmed, 1-40) in system_kv, an
injectable sync module with conflict/invalid revert and a pending-sync
flag, a background SystemNameSync worker (retries on boot and on a timer),
and api_client support for PUT /licenses/{key}/system-name plus the
additive systemName field/systemNameStatus on license validation. The
license service sends the stored name and applies the status. Covers
validation, pending retry, and conflict revert with a faked HTTP layer.

src/app_state.h

@@ -7,6 +7,7 @@
 #include "db/database_manager.h"
 #include "network/network_manager.h"
 #include "network/license_status.h"
+#include "network/system_name_sync.h"
 #include "audio/audio_manager.h"
 #include "audio/audio_downloader.h"
 
@@ -19,6 +20,7 @@ struct AppState {
     radiantwave::db::DatabaseManager db;
     radiantwave::network::NetworkManager network;
     radiantwave::license::LicenseStatusTracker licenseTracker;
+    radiantwave::systemname::SystemNameSync systemNameSync;
     radiantwave::audio::AudioManager audio;
     radiantwave::audio::AudioDownloader audioDownloader;
     bool quitRequested = false;