feat(front): 前台默认浅色模式并新增居中投稿按钮与实时空闲档期

- 主题: useTheme 默认 ModernLight + variables.css SSR-safe 浅值并精修组件对比度
- 首页: 插入居中投稿 CTA + 紧邻档期概览卡片(今日排期/待播歌曲/空闲档期数)
- 数据: useSongs 新增 fetchExpectedPlayDates 接入空闲档期并走 revision 轮询实时刷新
- 视觉: 移动端优先，少圆角硬边几何 + 概览卡片 3→1 列响应式

app/composables/useSongs.ts

@@ -17,6 +17,7 @@ export const useSongs = () => {
   const playTimes = ref<PlayTime[]>([])
   const playTimeEnabled = ref(false)
   const songCount = ref(0)
+  const expectedPlayDates = ref<any[]>([])
 
   // 显示通知
   const showNotification = (message: string, type: 'success' | 'error' | 'info' = 'info') => {
@@ -916,6 +917,56 @@ export const useSongs = () => {
     }
   }
 
+  // 获取期望播出日期及空闲档期
+  const fetchExpectedPlayDates = async (silent = false, forceRefresh = false) => {
+    if (!silent) {
+      loading.value = true
+    }
+    try {
+      const data = await dedup.dedupedRequest(
+        'expected-play-dates',
+        async () => {
+          const url = forceRefresh
+            ? `/api/expected-play-dates?t=${Date.now()}`
+            : '/api/expected-play-dates'
+          return await $fetch(url)
+        },
+        undefined,
+        forceRefresh
+      )
+      if (Array.isArray(data)) {
+        expectedPlayDates.value = data
+      } else if (data && Array.isArray((data as any).data)) {
+        expectedPlayDates.value = (data as any).data
+      } else {
+        expectedPlayDates.value = []
+      }
+      return expectedPlayDates.value
+    } catch (err: any) {
+      if (!silent) {
+        error.value = err.message || '获取档期数据失败'
+      }
+      return expectedPlayDates.value
+    } finally {
+      if (!silent) {
+        loading.value = false
+      }
+    }
+  }
+
+  const totalFreeSlots = computed(() =>
+    expectedPlayDates.value.reduce(
+      (sum, d) => sum + (Array.isArray(d.freeSequences) ? d.freeSequences.length : 0),
+      0
+    )
+  )
+
+  const openDaysCount = computed(() =>
+    expectedPlayDates.value.filter((d) => !d.isFull).length
+  )
+
+  const expectedPlayDatesLoaded = computed(() => Array.isArray(expectedPlayDates.value))
+
   // 初始化加载
   const initialize = async () => {
     await fetchPlayTimes()
@@ -932,6 +983,10 @@ export const useSongs = () => {
     publicSchedules,
     visibleSongs,
     songCount,
+    expectedPlayDates,
+    totalFreeSlots,
+    openDaysCount,
+    expectedPlayDatesLoaded,
     loading,
     error,
     notification,
@@ -943,6 +998,7 @@ export const useSongs = () => {
     fetchPublicSchedules,
     fetchPlayTimes,
     fetchSongCount,
+    fetchExpectedPlayDates,
     refreshSongsSilent,
     refreshSchedulesSilent,
     checkSimilarSongs,