style: 移除 emoji, 注释改为中文 (保留专业名词)

- 移除所有 Vue 组件和 README 中的 emoji (按钮文字、标题、功能列表)
- 将 src 下所有英文注释改为中文, 保留专业名词 (RRULE, EXDATE, ICS 等)
- 涉及文件: date.js, event-factory.js, ical-io.js, organize.js,
  recur.js, split.js, weekday.js, useCalendar.js, EventDetail.vue,
  App.vue, DropZone.vue, EventList.vue, HeaderBar.vue, README.md

src/components/HeaderBar.vue

@@ -41,13 +41,13 @@ function onDownload() {
 <template>
   <div class="header-actions">
     <button class="btn btn-primary" @click="onOrganize" :disabled="state.events.length === 0">
-      🔁 整理去重
+      整理去重
     </button>
-    <button class="btn btn-secondary" @click="undo" :disabled="!canUndo">↶ 撤销</button>
-    <button class="btn btn-secondary" @click="redo" :disabled="!canRedo">↷ 重做</button>
-    <button class="btn btn-secondary" @click="onImport">📥 导入文件</button>
+    <button class="btn btn-secondary" @click="undo" :disabled="!canUndo">撤销</button>
+    <button class="btn btn-secondary" @click="redo" :disabled="!canRedo">重做</button>
+    <button class="btn btn-secondary" @click="onImport">导入文件</button>
     <button class="btn btn-success" @click="onDownload" :disabled="state.events.length === 0">
-      ⬇ 下载 ICS
+      下载 ICS
     </button>
     <span v-if="state.dirty" class="dirty-dot" title="有未保存更改">●</span>
     <input ref="fileInput" type="file" accept=".ics" @change="onFilePicked" hidden />