fix(tcp): drop redundant _online write in the task to remove a race (greptile)

task_loop() set `_online = true` during the CONNECTING window, which races with
loop()'s `_online = false` on the main loop (plain bool, no synchronizes-with).
It's redundant: the main loop sets `_online = true` when it observes CONNECTED.
Removing it eliminates the race with no behaviour change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UWZuYkHBRqNb6BZHV8sTG5

src/TCPClientInterface.cpp

@@ -290,7 +290,9 @@ void TCPClientInterface::task_loop() {
                     if (connect()) {
                         _frame_buffer.clear();
                         _last_data_received = millis();
-                        _online = true;
+                        // _online is owned by the main loop (it sets it on
+                        // observing CONNECTED); writing it here would race with
+                        // loop()'s `_online = false` during the CONNECTING window.
                         _reconnected.store(true);       // main loop announces
                         _conn_state.store(CONNECTED);   // hand _client to main loop
                     } else {