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/default.nix

@@ -1,7 +1,7 @@
 _: {
   nixpkgs.config.allowUnfree = true;
   imports = [
-    ./fluxer
+    ./appimages
     ./essentials.nix
     ./packages.nix
     ./programs.nix