fix: add flake.lock back and update host configs

- Added `flake.lock` to pin dependencies (nixpkgs, home-manager, nixvim, flake-utils, etc.)
- Cleaned up `.gitignore`: stop ignoring `*.lock`
- Removed unused `flake-utils.inputs.nixpkgs.follows`
- Nixstation host:
  - Removed `keychron.nix`, Wacom tablet config, and redundant session variables
  - Simplified GDM/GNOME configuration and XKB layout
  - Commented out virtualisation setup
- Common NVIDIA host config:
  - Added explicit kernel modules, parameters, blacklists, and kernel version
  - Ensured GNOME + GDM configuration present
  - Updated NVIDIA driver settings and CUDA support

.gitignore

@@ -6,7 +6,6 @@
 *.old
 *.orig
 *.log
-*.lock
 .DS_Store
 Thumbs.db
 ehthumbs.db
@@ -95,4 +94,4 @@ custom/*/result
 custom/*/result/*
 **/.zig-cache/
 result/*
-result/
+result/