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>

lyrathorpe/home/sway.nix

@@ -237,6 +237,9 @@ in
     # text). Only colors-dark is needed; we never set initial-color-theme=light.
     settings = {
       main = {
+        # Nerd Font: monospace plus the powerline/Nerd glyphs the tmux
+        # statusline uses (otherwise they render as blank/"?").
+        font = "JetBrainsMono Nerd Font:size=11";
         # Advertise as xterm-256color so remote hosts without foot's terminfo
         # still behave (tmux re-adds foot's RGB/sync/etc. features -- see
         # shell.nix). The [main] section is the unheadered top of foot.ini.