Implement floating windows with pointer and keyboard controls

Add interactive move/resize operations using configurable pointer bindings
(Mod4+BTN_LEFT to move, Mod4+BTN_RIGHT to resize). Tiled windows
automatically float when dragged or resized.

Add keyboard commands for floating windows:
- move_up/down/left/right: move by pixel amount
- resize_width/height: resize by pixel amount
- swap_next/swap_prev: swap position in window stack

Fix float dimension initialization when windows first become floating,
and fix clamp crash when resizing windows larger than output bounds.

Update example config with documented keybinds and new pointer_binds block.

src/WindowManager.zig

@@ -110,6 +110,28 @@ fn manage_start(wm: *WindowManager) void {
             std.debug.assert(keybind.keysym != null);
             context.xkb_bindings.addBinding(river_seat_v1, keybind.keysym.?, keybind.modifiers, keybind.command);
         }
+
+        // Pointer bindings
+        for (context.config.pointer_binds.items) |pointer_bind| {
+            const binding = river_seat_v1.getPointerBinding(pointer_bind.button, pointer_bind.modifiers) catch {
+                log.err("Failed to create pointer binding", .{});
+                continue;
+            };
+
+            switch (pointer_bind.action) {
+                .move_window => {
+                    if (seat.move_pointer_binding) |old| old.destroy();
+                    binding.setListener(*Seat, Seat.movePointerBindingListener, seat);
+                    seat.move_pointer_binding = binding;
+                },
+                .resize_window => {
+                    if (seat.resize_pointer_binding) |old| old.destroy();
+                    binding.setListener(*Seat, Seat.resizePointerBindingListener, seat);
+                    seat.resize_pointer_binding = binding;
+                },
+            }
+            binding.enable();
+        }
     }
 
     {