Add declarative Flatpak support via nix-flatpak

Integrates the nix-flatpak module for declarative Flatpak management. Enables Flatpak system-wide, configures Home Manager to manage Flatpak apps, and adds an example Flatpak package with automatic weekly updates.

home/home.nix

@@ -154,6 +154,18 @@
   #   platformTheme.name = "gtk";
   # };
 
+  # Declarative Flatpak packages
+  services.flatpak = {
+    packages = [
+      "io.github.zen_browser.zen"
+      # Add more Flatpak apps here
+    ];
+    update.auto = {
+      enable = true;
+      onCalendar = "weekly";
+    };
+  };
+
   # Let Home Manager manage itself
   programs.home-manager.enable = true;