refactor: 移除重做/应用按钮, 自动应用更改, 删除死代码, 修复性能问题

UI 变更:
- 移除重做按钮及 redo/canRedo/redoStack 相关逻辑 (useCalendar, HeaderBar)
- 移除应用更改按钮, EventDetail 改为 watch form 自动应用 (deep watch)
- 用 syncing 标志防止 syncForm 回写时触发自动应用的循环

死代码清理:
- weekday.js: 移除未使用的 BYDAY_TO_INDEX 和 dowLabel
- date.js: 移除未使用的 toHHMM
- App.vue: 移除未使用的 .empty CSS
- DropZone.vue: 移除空的 .icon 容器及 CSS
- test/weekday.test.js: 移除对应死代码的 2 个测试用例

性能修复:
- WeekPreview: layoutEvents 从模板内联调用改为 computed (laidOutByDate),
  避免每次渲染重新计算布局
- MonthPreview: occByDate 过滤为仅网格内 42 天, 避免展开全部 occurrence

代码质量:
- useCalendar: prevMonth/nextMonth 合并为 shiftMonth(delta), 消除重复
- WeekPreview/MonthPreview: 移除未使用导入 (toISODate, toDate)

src/composables/useCalendar.js

@@ -3,7 +3,7 @@ import { parseICS, serializeICS } from '../lib/ical-io.js'
 import { organize as organizeEvents } from '../lib/organize.js'
 import { createEvent, cloneEvent } from '../lib/event-factory.js'
 import { splitRecurringEvent } from '../lib/split.js'
-import { todayISO, addDays, startOfMonth, startOfWeek, parseISODate, pad2 } from '../lib/date.js'
+import { todayISO, addDays, startOfMonth, parseISODate, pad2 } from '../lib/date.js'
 
 const MAX_HISTORY = 50
 
@@ -13,7 +13,6 @@ const state = reactive({
   events: [],
   selectedUid: null,
   history: [],
-  redoStack: [],
   fileName: null,
   dirty: false,
   viewMode: 'edit',        // 'edit' | 'month' | 'week'
@@ -34,7 +33,6 @@ function snapshot() {
 function pushHistory() {
   state.history.push(snapshot())
   if (state.history.length > MAX_HISTORY) state.history.shift()
-  state.redoStack = []
 }
 
 function restore(snap) {
@@ -44,7 +42,7 @@ function restore(snap) {
 }
 
 // ------------------------------------------------------------------ //
-// 操作
+// 文件加载
 // ------------------------------------------------------------------ //
 function loadFile(file) {
   return file.text().then((text) => {
@@ -53,12 +51,14 @@ function loadFile(file) {
     state.events = parsed.events
     state.selectedUid = null
     state.history = []
-    state.redoStack = []
     state.fileName = file.name
     state.dirty = false
   })
 }
 
+// ------------------------------------------------------------------ //
+// 事件操作
+// ------------------------------------------------------------------ //
 function selectEvent(uid) {
   state.selectedUid = uid
 }
@@ -72,6 +72,10 @@ function organize() {
   return result.stats
 }
 
+/**
+ * 更新指定事件的字段 (自动记录历史)。
+ * 供 EventDetail 的自动应用 (watch) 调用。
+ */
 function updateEvent(uid, patch) {
   pushHistory()
   const ev = state.events.find((e) => e.uid === uid)
@@ -127,16 +131,12 @@ function toggleOccurrence(uid, date) {
 
 function undo() {
   if (state.history.length === 0) return
-  state.redoStack.push(snapshot())
   restore(state.history.pop())
 }
 
-function redo() {
-  if (state.redoStack.length === 0) return
-  state.history.push(snapshot())
-  restore(state.redoStack.pop())
-}
-
+// ------------------------------------------------------------------ //
+// 序列化
+// ------------------------------------------------------------------ //
 function serialize() {
   return serializeICS({ meta: state.meta, events: state.events })
 }
@@ -152,16 +152,18 @@ function setViewMode(mode) {
   state.viewMode = mode
 }
 
-function prevMonth() {
+function shiftMonth(delta) {
   const { year, month } = parseISODate(startOfMonth(state.previewDate))
-  const d = new Date(year, month - 2, 1) // month-2 = 上个月 (0 基)
+  const d = new Date(year, month - 1 + delta, 1)
   state.previewDate = `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-01`
 }
 
+function prevMonth() {
+  shiftMonth(-1)
+}
+
 function nextMonth() {
-  const { year, month } = parseISODate(startOfMonth(state.previewDate))
-  const d = new Date(year, month, 1) // month = 下个月 (0 基)
-  state.previewDate = `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-01`
+  shiftMonth(1)
 }
 
 function prevWeek() {
@@ -180,7 +182,6 @@ const selectedEvent = computed(() =>
 )
 
 const canUndo = computed(() => state.history.length > 0)
-const canRedo = computed(() => state.redoStack.length > 0)
 const isLoaded = computed(() => state.events.length > 0)
 const stats = computed(() => {
   const nRecur = state.events.filter((e) => e.rrule !== null).length
@@ -192,7 +193,6 @@ export function useCalendar() {
     state,
     selectedEvent,
     canUndo,
-    canRedo,
     isLoaded,
     stats,
     loadFile,
@@ -204,7 +204,6 @@ export function useCalendar() {
     splitEvent,
     toggleOccurrence,
     undo,
-    redo,
     serialize,
     markSaved,
     setViewMode,