Fix UI freeze every 5 s

The settings main page exists from startup inside the closed dialog and
its IP timer always ran: networkInfo() spawns nmcli twice synchronously on
the GUI thread (~200 ms), stalling every animation. Poll only while the
page is visible. Also write the qt.alive heartbeat without QSaveFile's
fsync (bt-update only checks its mtime).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qml/settings/MainPage.qml

@@ -42,9 +42,14 @@ Item {
         refreshIp()
         refreshBrightness()
     }
+    // networkInfo() runs nmcli synchronously on the GUI thread (~200 ms), so
+    // poll only while this page is on screen. The page exists from startup
+    // inside the closed settings dialog; an always-running timer froze all
+    // animations every 5 s.
+    onVisibleChanged: if (visible) refreshIp()
     Timer {
         interval: 5000
-        running: true
+        running: page.visible
         repeat: true
         onTriggered: page.refreshIp()
     }