Phase 2: snapshot - window tree + screenshot via xcap

- Add xcap and image dependencies
- DesktopBackend trait with all 16 methods for future extensibility
- X11Backend with real snapshot() using xcap Window::all() and
  Monitor::all() for z-ordered window enumeration and screenshot
- Stub implementations for input/window management (phases 4-6)
- Wire X11Backend into DaemonState (now returns Result)
- Real snapshot handler replacing placeholder, updates ref map

src/main.rs

@@ -1,3 +1,4 @@
+mod backend;
 mod cli;
 mod core;
 mod daemon;