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>

Makefile

@@ -5,9 +5,9 @@
 #   make clean    — remove build artifacts
 #   make iso      — build the bootable ISO (requires Arch Linux + archiso)
 
-.PHONY: all wm deskbar launcher installer clean install iso
+.PHONY: all wm deskbar launcher installer nebula clean install iso
 
-all: wm deskbar launcher installer
+all: wm deskbar launcher installer nebula
 
 wm:
 	$(MAKE) -C wm
@@ -21,11 +21,15 @@ launcher:
 installer:
 	$(MAKE) -C installer
 
+nebula:
+	$(MAKE) -C nebula
+
 clean:
 	$(MAKE) -C wm        clean
 	$(MAKE) -C deskbar   clean
 	$(MAKE) -C launcher  clean
 	$(MAKE) -C installer clean
+	$(MAKE) -C nebula    clean
 
 install: all
 	# binaries
@@ -33,6 +37,7 @@ install: all
 	install -Dm755 deskbar/oriondeskbar    /usr/local/bin/oriondeskbar
 	install -Dm755 launcher/orionlauncher  /usr/local/bin/orionlauncher
 	install -Dm755 themes/orionwallpaper   /usr/local/bin/orionwallpaper
+	install -Dm755 nebula/nebula           /usr/local/bin/nebula
 	# GTK theme
 	mkdir -p /usr/share/themes
 	cp -r themes/OrionOS /usr/share/themes/