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/daemon/handler.rs

@@ -1,31 +1,52 @@
 use std::sync::Arc;
 use tokio::sync::Mutex;
 
+use crate::backend::DesktopBackend;
 use crate::core::protocol::{Request, Response};
+use crate::core::refs::RefEntry;
 use super::state::DaemonState;
 
 pub async fn handle_request(
     request: &Request,
-    _state: &Arc<Mutex<DaemonState>>,
+    state: &Arc<Mutex<DaemonState>>,
 ) -> Response {
     match request.action.as_str() {
-        "snapshot" => {
-            Response::ok(serde_json::json!({
-                "screenshot": "/tmp/desktop-ctl-placeholder.png",
-                "windows": [
-                    {
-                        "ref_id": "w1",
-                        "xcb_id": 0,
-                        "title": "Placeholder Window",
-                        "app_name": "placeholder",
-                        "x": 0, "y": 0, "width": 1920, "height": 1080,
-                        "focused": true, "minimized": false
-                    }
-                ]
-            }))
-        }
-        action => {
-            Response::err(format!("Unknown action: {action}"))
-        }
+        "snapshot" => handle_snapshot(request, state).await,
+        action => Response::err(format!("Unknown action: {action}")),
+    }
+}
+
+async fn handle_snapshot(
+    request: &Request,
+    state: &Arc<Mutex<DaemonState>>,
+) -> Response {
+    let annotate = request.extra.get("annotate")
+        .and_then(|v| v.as_bool())
+        .unwrap_or(false);
+
+    let mut state = state.lock().await;
+
+    match state.backend.snapshot(annotate) {
+        Ok(snapshot) => {
+            // Update ref map
+            state.ref_map.clear();
+            for win in &snapshot.windows {
+                state.ref_map.insert(RefEntry {
+                    xcb_id: win.xcb_id,
+                    app_class: win.app_name.clone(),
+                    title: win.title.clone(),
+                    pid: 0, // xcap doesn't expose PID directly in snapshot
+                    x: win.x,
+                    y: win.y,
+                    width: win.width,
+                    height: win.height,
+                    focused: win.focused,
+                    minimized: win.minimized,
+                });
+            }
+
+            Response::ok(serde_json::to_value(&snapshot).unwrap_or_default())
+        }
+        Err(e) => Response::err(format!("Snapshot failed: {e}")),
     }
 }