fix: Android event handler lost after restart — g_event_handler now preserved

Root cause: instance_lite_restart was creating new thread without
re-setting g_event_handler, causing all chat events (CHANNEL_UPDATED,
CONNECT_RESULT, etc.) to be silently dropped after app restart.

Fixes:
- instance_lite_restart: save/restore g_event_handler across hard restart
- instance_thread restart loop: re-call chat_event_set_handler after uasync_create
- utun_bridge_restart: call instance_lite_set_event_handler before restart
- AppEventHandler SharedFlow replay: 1 → 16 to not lose early init events
- ViewModel: added diagnostic logging for channel/connect events
- Keep diagnostic logs in instance_lite (set_handler, chat_event_forward NULL check)
- instance_event_handler: removed static for cross-block forward declaration

src/chat/chat_event.c

@@ -8,9 +8,17 @@
 #include "chat_event.h"
 #include "../lib/debug_config.h"
 
+#ifdef __ANDROID__
+#include <android/log.h>
+#endif
+
 static chat_event_handler_fn g_handler = NULL;
 
 void chat_event_set_handler(chat_event_handler_fn handler) {
+#ifdef __ANDROID__
+    __android_log_print(ANDROID_LOG_INFO, "utun-evt", "chat_event_set_handler: handler=%p old=%p",
+                        (void*)handler, (void*)g_handler);
+#endif
     g_handler = handler;
 }