feat(bar): 新增绑定模式指示器显示当前 binding mode

- 新增 bar/binding: 监听 binding mode 变更，显示当前模式名
- 支持点击单元循环切换绑定模式（binding_mode_cycle）
- 默认模式下按 show_default 配置决定是否隐藏单元
- 补充 binding 相关默认配置（颜色、内边距、是否显示默认模式）

src/bar/binding.c

@@ -0,0 +1,104 @@
+#include "bar/binding.h"
+
+#include <assert.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <zdwm/action.h>
+#include <zdwm/bar.h>
+#include <zdwm/listeners.h>
+#include <zdwm/types.h>
+
+#include "base/memory.h"
+#include "core/listeners.h"
+
+typedef struct bar_binding_state_t {
+  zdwm_binding_mode_notify_t mode;
+
+  bar_binding_config_t config;
+
+  bool inited;
+  bool dirty;
+} bar_binding_state_t;
+
+static void *
+bar_binding_create_state(zdwm_output_id_t output_id, void *config) {
+  const bar_binding_config_t *bar_config = config;
+  assert(bar_config);
+
+  auto state    = p_new(bar_binding_state_t, 1);
+  state->config = *bar_config;
+
+  return state;
+}
+
+static void bar_binding_update(
+  zdwm_bar_item_t *item,
+  const zdwm_bar_cell_api_t *cells,
+  void *state
+) {
+  auto data = (bar_binding_state_t *)state;
+  if (!data->inited || !data->dirty) return;
+
+  auto config = &data->config;
+  auto mode   = &data->mode;
+
+  if (!config->show_default && mode->is_default_mode) {
+    cells->set_cell_count(item, 0);
+  } else {
+    constexpr auto index = 0;
+    cells->set_cell_count(item, 1);
+    cells->cell_set_text(item, index, mode->name);
+    cells->cell_set_bg(item, index, config->bg);
+    cells->cell_set_fg(item, index, config->fg);
+  }
+
+  data->dirty = false;
+}
+
+static zdwm_action_t bar_binding_on_click(
+  zdwm_bar_item_t *item,
+  size_t cell_index,
+  int32_t x,
+  void *state
+) {
+  return (zdwm_action_t){
+    .type                  = ZDWM_ACTION_BINDING_MODE_CYCLE,
+    .as.binding_mode_cycle = {.delta = 1},
+  };
+}
+
+static void bar_binding_destroy_state(void *state) {
+  if (!state) return;
+
+  auto data = (bar_binding_state_t *)state;
+  p_delete(&data);
+}
+
+zdwm_bar_item_type_t bar_binding = {
+  .create_state       = bar_binding_create_state,
+  .update             = bar_binding_update,
+  .on_click           = bar_binding_on_click,
+  .destroy_state      = bar_binding_destroy_state,
+  .update_interval_ms = 0,
+};
+
+static void
+bar_binding_mode_notify(zdwm_binding_mode_notify_t mode, void *user_data) {
+  auto state = (bar_binding_state_t *)user_data;
+  if (!state->inited) {
+    state->mode = mode;
+
+    state->inited = true;
+    state->dirty  = true;
+    return;
+  }
+
+  if (state->mode.id == mode.id) return;
+  state->mode = mode;
+
+  state->dirty = true;
+}
+
+void bar_binding_add_listeners(listeners_t *listeners, void *state) {
+  listeners_add_binding_mode_notify(listeners, bar_binding_mode_notify, state);
+}