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

modules/core/packages/appimages/helium.nix

@@ -4,8 +4,8 @@
   fetchurl,
 }: let
   pname = "helium";
-  version = "0.15.7.1";
-  hash = "sha256-+zGEGfhIiZWE8mUYb3HrkoM7reFBMdfXlgw3KWT0T98=";
+  version = "0.16.2.1";
+  hash = "sha256-LS3sjyzR1ySfD0L9RulF5Zgys9FDLm8APYR/AJcKDQI=";
 
   src = fetchurl {
     url = "https://github.com/imputnet/helium-linux/releases/download/${version}/helium-${version}-x86_64.AppImage";

modules/core/packages/essentials.nix

@@ -8,7 +8,7 @@
     sl
     fortune
     yt-dlp
-    pdfium-binaries
+    pdfium
     bitwarden-cli
     xdg-utils
     gcc