Add Helium AppImage and replace Chromium webapps

- Add Helium browser as an AppImage and include it in system packages.
- Rename fluxer package into the appimages module and update imports.
- Add `SUPER+H` keybinding to launch Helium and `SUPER+CONTROL+T` for
  tsukimi.
- Change web‑app desktop entries to use Helium instead of Chromium.
- Disable ungoogled‑chromium and ChatGPT modules on the laptop host.
- Fix a minor typo in a comment within hardware.nix.

modules/core/packages/appimages/default.nix

@@ -0,0 +1,6 @@
+{pkgs, ...}: let
+  fluxer = pkgs.callPackage ./fluxer.nix {};
+  helium = pkgs.callPackage ./helium.nix {};
+in {
+  environment.systemPackages = [fluxer helium];
+}