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

Cargo.toml

@@ -15,3 +15,5 @@ anyhow = "1"
 dirs = "6"
 libc = "0.2"
 uuid = { version = "1", features = ["v4"] }
+xcap = "0.8"
+image = { version = "0.25", features = ["png"] }