add DEBUG_CATEGORY_CONNECTIVITY for handshake/protocol lifecycle logs

New category (id=25) replaces DEBUG_CATEGORY_DEBUG in chat_sync and
chat_core. This allows enabling only protocol-level logs (SEND/RECV,
timeouts, handshake) without the noise from DEBUG category.

Changes:
- debug_config.h: add CONNECTIVITY=25, COUNT→26
- debug_config.c: add 'connectivity' to g_debug_categories
- chat_sync.c: DEBUG→CONNECTIVITY in all protocol logs
- chat_core.c: DEBUG→CONNECTIVITY in connection/create logs
- utun_node.cpp: remove hardcoded DEBUG=TRACE (now config-driven)
- chatgui.cfg: connectivity=info in debug_categories

lib/debug_config.c

@@ -129,6 +129,7 @@ static const struct {
     {"etcp_route", DEBUG_CATEGORY_ETCPROUTE},
     {"bbr", DEBUG_CATEGORY_BBR},
     {"etcp_dump", DEBUG_CATEGORY_ETCP_DUMP},
+    {"connectivity", DEBUG_CATEGORY_CONNECTIVITY},
     {"all", DEBUG_CATEGORY_ALL},
     {NULL, DEBUG_CATEGORY_NONE}
 };