fix(theme): use '昼' instead of '日', improve disabled text contrast

- App.vue: change toggle label from '日' to '昼' (proper pairing with '夜')
- song-theme.css: light mode --song-text-disabled #9aa7ad → #7e8f98 for better visibility

src/App.vue

@@ -45,7 +45,7 @@ async function handleLogout() {
         type="button"
         :title="theme.isDark ? '切至天青昼' : '切至黛墨夜'"
         @click="theme.toggle()"
-      >{{ theme.isDark ? '日' : '夜' }}</button>
+      >{{ theme.isDark ? '昼' : '夜' }}</button>
     </header>
     <main class="app-main" :class="{ 'app-main--admin': isAdminRoute }">
       <router-view v-slot="{ Component }">

src/styles/song-theme.css

@@ -12,7 +12,7 @@
 
   --song-text: #3a4a52;
   --song-text-secondary: #5a6b74;
-  --song-text-disabled: #9aa7ad;
+  --song-text-disabled: #7e8f98;
 
   --song-primary: #4f78b8;
   --song-primary-bg: #e7eef9;