feat(fonts): JetBrains Mono Nerd Font on every host

The tmux statusline draws powerline/Nerd glyphs that default fonts lack,
so they render as blank/"?". tmux runs on every host (not just the Sway
ones), so install the font in the shared common-nixos module rather than
swaywm -- a future console-only or non-Sway host gets it too. The Mac
installs it via the Darwin config (/Library/Fonts). foot names it as its
main font (home/sway.nix).

On macOS, iTerm2's font is still a GUI setting: Settings -> Profiles ->
Text -> Font -> "JetBrainsMono Nerd Font".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/Darwin/configuration.nix

@@ -6,6 +6,11 @@
 {
   programs.zsh.enable = true;
 
+  # Install the Nerd Font into /Library/Fonts so iTerm2 can use it (set it in
+  # iTerm2 -> Settings -> Profiles -> Text -> Font: "JetBrainsMono Nerd Font").
+  # Provides the powerline/Nerd glyphs the tmux statusline draws.
+  fonts.packages = [ pkgs.nerd-fonts.jetbrains-mono ];
+
   # CLI tooling sourced from nixpkgs instead of Homebrew formulae. Pure library
   # dependencies are omitted; nix pulls them into closures automatically.
   environment.systemPackages = with pkgs; [