Add Jan UI and selective Flatpak options

- Expose Jan as a core package and add a Hyprland shortcut (Mod + J). -
Refactor Flatpak module to provide per‑package enable options; default
package list is now empty. - Update librewolf search template to use a
single‑parameter URL. - Minor comment formatting changes across host
module files.

modules/core/packages/packages.nix

@@ -25,6 +25,7 @@
     tutanota = tutanota-desktop;
     exercism = exercism;
     opencode = opencode;
+    jan = jan;
     lazygit = lazygit;
     prismlauncher = inputs.prismlauncher-cracked.packages.${system}.default;
     spaceCadetPinball = space-cadet-pinball;