nixos-module: use playwright's chromium-headless-shell instead of pkgs.chromium for PDF generation

snappdf only ever shells out to `$SNAPPDF_CHROMIUM_PATH --headless --print-to-pdf=...`,
so the full browser UI/GPU stack pkgs.chromium ships is unnecessary. Google's official
"Chrome for Testing" headless-only build (packaged by nixpkgs' playwright-driver,
selected via `browsers.override` with every other browser/ffmpeg disabled) does the
same job for a ~1.45GB smaller closure (431MB vs 1.88GB), needs no allowUnfree, and
PDF generation is noticeably faster in the VM test (~2.5s vs ~10s per request).

Drops the now-unnecessary allowUnfree wiring from the interactive VM and automated
test configs, which existed solely to build pkgs.chromium.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/default.nix

@@ -10,9 +10,6 @@
     (import ./common.nix { inherit proxyServer; })
   ];
 
-  # Required to build pkgs.chromium, which the module pulls in for PDF generation.
-  nixpkgs.config.allowUnfree = true;
-
   # `nix flake check` evaluates config.system.build.toplevel directly, which asserts a real root
   # filesystem and bootloader are configured. `just build-vm` builds config.system.build.vm
   # instead, where the qemu-vm module overrides both of these with a tmpfs root and direct