Add Nebula — native C settings panel (Display, Appearance, Input, About)

nebula/nebula.c (~600 lines, single-file, native Xlib/Xft):
- Display tab: resolution cycling via xrandr, DPI toggle (96/144),
  brightness control via brightnessctl
- Appearance tab: wallpaper reset/open, shows active GTK/icon theme
- Input tab: key repeat delay + rate steppers (reads live from XKB),
  tap-to-click and natural scrolling toggles via xinput
- About tab: version, component, platform info
- Fixed-size 700×500 window, dark-navy palette, sidebar nav with
  accent mark on selected tab, dirty-state indicator, Apply/Close
- Escape/Q/Enter shortcuts; Enter applies pending changes

Wired up:
- Root Makefile: nebula target, install -Dm755 nebula/nebula
- Right-click desktop menu: "System Settings" → nebula
- ISO: binary installed to /usr/local/bin/nebula, Super+S xbindkeys
  shortcut, file permissions map entry
- .gitignore: nebula/nebula binary

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

wm/src/menu.c

@@ -13,6 +13,7 @@ static const MenuItem MENU_ITEMS[] = {
     { "Web Browser",       "firefox &"                                  },
     { "System Monitor",    "alacritty -T 'System Monitor' -e htop &"    },
     { "Volume Control",    "pavucontrol &"                              },
+    { "System Settings",   "nebula &"                                   },
     { "Display Settings",  "arandr &"                                   },
     { "──────────────────", NULL                                         },
     { "Change Wallpaper",  "orionwallpaper --reset &"                    },