feat: 新增视图切换状态管理及颜色/日期工具

- date.js: 新增 startOfWeek, startOfMonth, hhmmToMinutes 辅助函数
- colors.js (新增): 基于 uid 哈希的事件颜色分配, 12 色调色板
- useCalendar.js: 新增 viewMode (edit/month/week) 和 previewDate 状态,
  以及 setViewMode/prevMonth/nextMonth/prevWeek/nextWeek 操作

src/composables/useCalendar.js

@@ -3,6 +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'
 
 const MAX_HISTORY = 50
 
@@ -15,6 +16,8 @@ const state = reactive({
   redoStack: [],
   fileName: null,
   dirty: false,
+  viewMode: 'edit',        // 'edit' | 'month' | 'week'
+  previewDate: todayISO(),  // 预览锚定日期 (月预览取当月, 周预览取当周)
 })
 
 // ------------------------------------------------------------------ //
@@ -142,6 +145,33 @@ function markSaved() {
   state.dirty = false
 }
 
+// ------------------------------------------------------------------ //
+// 视图切换 / 预览导航
+// ------------------------------------------------------------------ //
+function setViewMode(mode) {
+  state.viewMode = mode
+}
+
+function prevMonth() {
+  const { year, month } = parseISODate(startOfMonth(state.previewDate))
+  const d = new Date(year, month - 2, 1) // month-2 = 上个月 (0 基)
+  state.previewDate = `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-01`
+}
+
+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`
+}
+
+function prevWeek() {
+  state.previewDate = addDays(state.previewDate, -7)
+}
+
+function nextWeek() {
+  state.previewDate = addDays(state.previewDate, 7)
+}
+
 // ------------------------------------------------------------------ //
 // 计算属性
 // ------------------------------------------------------------------ //
@@ -177,5 +207,10 @@ export function useCalendar() {
     redo,
     serialize,
     markSaved,
+    setViewMode,
+    prevMonth,
+    nextMonth,
+    prevWeek,
+    nextWeek,
   }
 }