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/essentials.nix

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