settings: skip unchanged saves so external edits are not swallowed (#3504)

saveSettings() set _selfWrite before setText(). When the serialized
settings were identical to the file, nothing was written, no
fileChanged followed, and the flag stayed set. The next external edit
of settings.json was then taken for our own write and never reloaded.

(cherry picked from commit 84594e6449)

quickshell/Common/SettingsData.qml

@@ -1988,8 +1988,11 @@ Singleton {
     function saveSettings() {
         if (_loading || _parseError || !_hasLoaded)
             return;
+        const json = JSON.stringify(Store.toJson(root), null, 2);
+        if (json === settingsFile.text())
+            return;
         _selfWrite = true;
-        settingsFile.setText(JSON.stringify(Store.toJson(root), null, 2));
+        settingsFile.setText(json);
         if (_isReadOnly)
             _checkSettingsWritable();
     }