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/modules/common-nixos.nix

@@ -12,4 +12,10 @@
     git
     fastfetch
   ];
+
+  # Terminal font with powerline/Nerd glyphs. Installed on every host because
+  # the tmux statusline (which uses these glyphs) runs everywhere, not just on
+  # the Sway/graphical hosts. foot names it explicitly (home/sway.nix); the Mac
+  # installs it via the Darwin config.
+  fonts.packages = [ pkgs.nerd-fonts.jetbrains-mono ];
 }