feat(config): parse effects.windows.shadows from Lua config

Add CfgEffectsWindowShadows struct (12 fields) and CfgEffectsWindows.shadows
member. Add wlr_scene_shadow *shadow field to Client struct for runtime
shadow node tracking. Update example/config.lua with full shadow config
block.

The config parser handles missing shadows table gracefully (all default).

include/client.h

@@ -14,6 +14,7 @@ struct wlr_layer_surface_v1;
 struct wlr_xdg_surface;
 struct wlr_xwayland_surface;
 struct wlr_xdg_toplevel_decoration_v1;
+struct wlr_scene_shadow;
 
 /* Client types */
 enum { XDGShell, LayerShell, X11 };
@@ -54,6 +55,7 @@ typedef struct Client {
 #endif
 	unsigned int bw;
 	int corner_radius;
+	struct wlr_scene_shadow *shadow;
 	uint32_t tags;
 	int isfloating, isurgent, isfullscreen, isscratchpad;
 	uint32_t resize; /* configure serial of a pending resize */