river-status: add mode event to seat status

This allows clients such as a status bar to display the currently active
mode.

build.zig

@@ -112,7 +112,7 @@ pub fn build(b: *zbs.Builder) !void {
     scanner.generate("zwp_pointer_constraints_v1", 1);
 
     scanner.generate("zriver_control_v1", 1);
-    scanner.generate("zriver_status_manager_v1", 2);
+    scanner.generate("zriver_status_manager_v1", 3);
     scanner.generate("river_layout_manager_v3", 1);
 
     scanner.generate("zwlr_layer_shell_v1", 4);