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/home/xdg/desktop-entries.nix

@@ -12,7 +12,7 @@
   makeEntry = app: {
     name = app.displayName;
     genericName = app.genericName or app.displayName;
-    exec = "chromium --profile-directory=Default --app=${app.url}";
+    exec = "helium --profile-directory=Default --app=${app.url}";
     icon = "${fetchIcon app.iconUrl app.iconSha}";
     terminal = false;
     type = "Application";