Update flake.lock, bump Helium, replace pdfium

- Refresh flake.lock with newer revisions and remove the unused fenix
  input
- Disable OBS Studio in Garand‑Desktop home modules
- Bump Helium AppImage to 0.16.2.1 and update its hash
- Switch from pdfium‑binaries to the pdfium package

hosts/Garand-Desktop/home-modules.nix

@@ -41,7 +41,7 @@ _: {
   /*
   Media recording and streaming
   */
-  obs-studio.enable = true; # OBS Studio: streaming and recording software
+  obs-studio.enable = false; # OBS Studio: streaming and recording software
 
   /*
   Office suite