feat(browsers): add firefox

- add nur
- set firefox as main browser
- move browsers into separate modules

home/browsers/chromium.nix

@@ -0,0 +1,11 @@
+{ config, pkgs, ... }:
+
+{
+  programs.chromium = {
+    enable = true;
+    package = pkgs.chromium.override {
+      enableVaapi = true; # NVIDIA also requires vdpau backend
+      commandLineArgs = "--force-dark-mode";
+    };
+  };
+}