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/Modules/Greetd/GreeterUserTheme.qml

@@ -0,0 +1,51 @@
+pragma Singleton
+pragma ComponentBehavior: Bound
+
+import QtQuick
+import Quickshell
+import qs.Common
+import qs.Services
+
+Singleton {
+    id: root
+
+    readonly property var log: Log.scoped("GreeterUserTheme")
+    readonly property string greetCfgDir: Quickshell.env("DMS_GREET_CFG_DIR") || "/var/cache/dms-greeter"
+
+    property string activeUsername: ""
+
+    function userCacheDir(username) {
+        if (!username)
+            return "";
+        return greetCfgDir + "/users/" + username;
+    }
+
+    function applyForUser(username) {
+        const name = (username || "").trim();
+        activeUsername = name;
+        if (!name) {
+            applyDefault();
+            return;
+        }
+        const dir = userCacheDir(name);
+        if (typeof GreeterUsersService !== "undefined" && GreeterUsersService.hasSyncedTheme(name)) {
+            Theme.setGreeterColorsBaseDir(dir);
+            SessionData.setGreeterSessionBaseDir(dir);
+            GreetdSettings.setConfigBaseDir(dir);
+            return;
+        }
+        applyDefault();
+    }
+
+    function applyDefault() {
+        activeUsername = "";
+        Theme.resetGreeterColorsBaseDir();
+        SessionData.resetGreeterSessionBaseDir();
+        GreetdSettings.resetConfigBaseDir();
+    }
+
+    readonly property string activeWallpaperOverridePath: {
+        const base = activeUsername && typeof GreeterUsersService !== "undefined" && GreeterUsersService.hasSyncedTheme(activeUsername) ? userCacheDir(activeUsername) : greetCfgDir;
+        return base ? base + "/greeter_wallpaper_override.jpg" : "";
+    }
+}