fix(ui): 适配前台浅色背景细节

- 学校 logo 替换为黑字版本，修改 useSiteConfig 默认值和 fallback
- 首页通知卡片补齐浅色样式：status-tag（三态）、unread-indicator、
  invite-actions、accept-btn、reject-btn
- 投稿区域状态徽章由绝对定位改为搜索标题行内布局，背景改为浅色
- 消息设置页面整体改为浅色主题，主色系改为 indigo 以匹配 ModernLight

app/composables/useSiteConfig.js

@@ -52,7 +52,7 @@ export const useSiteConfig = () => {
       siteConfig.value = {
         siteTitle: '校园广播站点歌系统',
         siteLogoUrl: '/images/site-logo.png',
-        schoolLogoHomeUrl: '/images/school-logo-white.png',
+        schoolLogoHomeUrl: '/images/school-logo-black.png',
         schoolLogoPrintUrl: '/images/school-logo-transparent.png',
         siteDescription: '校园广播站点歌系统 - 让你的声音被听见',
         siteVersion: '',
@@ -81,7 +81,7 @@ export const useSiteConfig = () => {
   })
   const schoolLogoHomeUrl = computed(() => {
     const raw = (siteConfig.value.schoolLogoHomeUrl || '').trim()
-    const fallback = '/images/school-logo-white.png'
+    const fallback = '/images/school-logo-black.png'
 
     if (!raw) return fallback
     if (raw === '/images/school-logo-white.png' || raw === '/images/school-logo-black.png') return fallback