Add on-screen display, sidebar keybinds, function keys, gh package

- Add Osd.qml: themed top-center popup for volume/mic/brightness/media
  changes. Volume and mic react directly to Pipewire so they fire for
  any change, not just our own keybinds; brightness is notified
  explicitly (no live service exists for DDC/CI writes); media is
  driven by two playerctl --follow watchers (status + metadata) so
  track changes, pause/resume, and stop all show correctly without
  double-firing
- Add Super+Shift+Z/X to toggle the Left/RightBar sidebars, via a new
  Quickshell IpcHandler wired to the same expand/collapse logic as the
  BottomBar arrows
- Add function-key binds for volume up/down/mute, mic mute, brightness
  up/down, and media play/pause/stop/next/prev (wpctl/playerctl);
  extend brightness-ctl with relative up/down steps for both the
  backlight and DDC/CI paths
- Add gh and playerctl to packages

modules/apps.nix

@@ -36,6 +36,7 @@ in
     bibata-cursors            # Cursor theme(s) for the cursor picker
     xcur2png                  # Renders Xcursor files to PNG for the cursor picker's previews
     imagemagick                # `identify`, used to pick the right cursor-preview size
+    playerctl                  # MPRIS media control, for the function-key media binds
 
     # Apps
     blender
@@ -66,6 +67,7 @@ in
   # Global command-line utilities available everywhere
   environment.systemPackages = with pkgs; [
     git
+    gh # GitHub CLI — was only installed ad-hoc before (login didn't survive)
     vim
     fastfetch
     gparted