feat(Greeter): improved multi-user UI and per-user theme sync

- Introduce multi-account greeter login with per-user theme previews
- Add `dms greeter sync --profile` for secondary users with or without sudo
- Add Manage greeter group membership from Settings UI → Users Tab

quickshell/Common/SessionData.qml

@@ -1353,13 +1353,27 @@ Singleton {
         }
     }
 
+    readonly property string _greeterCacheDir: Quickshell.env("DMS_GREET_CFG_DIR") || "/var/cache/dms-greeter"
+
+    property string greeterSessionBaseDir: root._greeterCacheDir
+
+    function setGreeterSessionBaseDir(dir) {
+        const next = dir || root._greeterCacheDir;
+        if (greeterSessionBaseDir === next)
+            return;
+        greeterSessionBaseDir = next;
+        if (isGreeterMode)
+            greeterSessionFile.reload();
+    }
+
+    function resetGreeterSessionBaseDir() {
+        setGreeterSessionBaseDir(root._greeterCacheDir);
+    }
+
     FileView {
         id: greeterSessionFile
 
-        path: {
-            const greetCfgDir = Quickshell.env("DMS_GREET_CFG_DIR") || "/var/cache/dms-greeter";
-            return greetCfgDir + "/session.json";
-        }
+        path: root.greeterSessionBaseDir ? (root.greeterSessionBaseDir + "/session.json") : ""
         preload: isGreeterMode
         blockLoading: false
         blockWrites: true