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/backend/mod.rs

@@ -0,0 +1,58 @@
+pub mod x11;
+
+use anyhow::Result;
+use crate::core::types::Snapshot;
+
+#[allow(dead_code)]
+pub trait DesktopBackend: Send {
+    /// Capture a screenshot and return a z-ordered window tree with @wN refs.
+    fn snapshot(&mut self, annotate: bool) -> Result<Snapshot>;
+
+    /// Focus a window by its X11 window ID.
+    fn focus_window(&mut self, xcb_id: u32) -> Result<()>;
+
+    /// Move a window to absolute coordinates.
+    fn move_window(&mut self, xcb_id: u32, x: i32, y: i32) -> Result<()>;
+
+    /// Resize a window.
+    fn resize_window(&mut self, xcb_id: u32, w: u32, h: u32) -> Result<()>;
+
+    /// Close a window gracefully.
+    fn close_window(&mut self, xcb_id: u32) -> Result<()>;
+
+    /// Click at absolute coordinates.
+    fn click(&mut self, x: i32, y: i32) -> Result<()>;
+
+    /// Double-click at absolute coordinates.
+    fn dblclick(&mut self, x: i32, y: i32) -> Result<()>;
+
+    /// Type text into the focused window.
+    fn type_text(&mut self, text: &str) -> Result<()>;
+
+    /// Press a single key by name.
+    fn press_key(&mut self, key: &str) -> Result<()>;
+
+    /// Send a hotkey combination.
+    fn hotkey(&mut self, keys: &[String]) -> Result<()>;
+
+    /// Move the mouse cursor to absolute coordinates.
+    fn mouse_move(&mut self, x: i32, y: i32) -> Result<()>;
+
+    /// Scroll the mouse wheel.
+    fn scroll(&mut self, amount: i32, axis: &str) -> Result<()>;
+
+    /// Drag from one position to another.
+    fn drag(&mut self, x1: i32, y1: i32, x2: i32, y2: i32) -> Result<()>;
+
+    /// Get the screen resolution.
+    fn screen_size(&self) -> Result<(u32, u32)>;
+
+    /// Get the current mouse position.
+    fn mouse_position(&self) -> Result<(i32, i32)>;
+
+    /// Take a screenshot and save to a path (no window tree).
+    fn screenshot(&mut self, path: &str, annotate: bool) -> Result<String>;
+
+    /// Launch an application.
+    fn launch(&self, command: &str, args: &[String]) -> Result<u32>;
+}