fix(ui): 彻底修复深浅色主题切换及内部页面样式

- 恢复 variables.css 完整的 ClassicDark/ModernDark 等主题定义
- 恢复 nuxt.config.ts 的主题系统逻辑（默认改为 ClassicLight，保留切换能力）
- 修复 BackupManager / VotersModal / ProfileForm / LyricsModal / RequestForm / SongList / ScheduleList / StatsSlide / ArtistSlide 等数十处组件内部硬编码深色（#1a1a1a / #040e15 等）绕过主题的问题，全部改为使用 CSS 变量 (var(--bg-primary) 等)
- 这样实现了：在浅色模式下，所有页面均正确显示浅色；在深色模式下，又能正常呈现深色设计，且恢复了右上角的深浅色切换能力

nuxt.config.ts

@@ -194,10 +194,6 @@ export default defineNuxtConfig({
           name: 'x-default-theme',
           content: process.env.NUXT_PUBLIC_DEFAULT_THEME || 'ClassicLight'
         },
-        {
-          name: 'x-enabled-themes',
-          content: process.env.NUXT_PUBLIC_ENABLED_THEMES || 'ClassicLight,ModernLight'
-        },
         { name: 'apple-mobile-web-app-capable', content: 'yes' },
         { name: 'apple-mobile-web-app-status-bar-style', content: 'black-translucent' },
         { name: 'apple-mobile-web-app-title', content: 'VoiceHub管理' },
@@ -210,28 +206,18 @@ export default defineNuxtConfig({
           tagPosition: 'head',
           children: [
             `(function(){try{`,
-            `  var k='voicehub:theme', d=document, r=d.documentElement, s=null, eff=null, def=null, list=[];`,
-            `  try{`,
-            `    var mt=d.querySelector('meta[name="x-enabled-themes"]');`,
-            `    if(mt && mt.content){list=String(mt.content).split(',').map(function(x){return x.replace(/^\\s+|\\s+$/g,'')}).filter(Boolean)}`,
-            `  }catch(e){list=[]}`,
-            `  try{`,
-            `    var md=d.querySelector('meta[name="x-default-theme"]');`,
-            `    if(md && md.content){def=md.content}`,
-            `  }catch(e){def=null}`,
-            `  if(!def){def='ClassicLight'}`,
-            `  if(list.length===0){list=['ClassicLight','ModernLight']}`,
-            `  function valid(t){`,
-            `    if(list.indexOf(t)!==-1){return t}`,
-            `    if(list.indexOf(def)!==-1){return def}`,
-            `    return list[0]`,
-            `  }`,
+            `  var k='voicehub:theme', d=document, r=d.documentElement, s=null, eff=null;`,
             `  try{s=localStorage.getItem(k)}catch(e){s=null}`,
-            `  if(!s){s=def}`,
-            `  else{s=valid(s)}`,
-            `  if(typeof s==='string' && (s==='System'||s==='system')){eff=window.matchMedia('(prefers-color-scheme: dark)').matches?'dark':'light'}`,
+            `  if(!s){`,
+            `    try{`,
+            `      var meta=document.querySelector('meta[name="x-default-theme"]');`,
+            `      if(meta && meta.content){s=meta.content}`,
+            `    }catch(e){s=null}`,
+            `    if(!s){s='ClassicLight'}`,
+            `  }`,
+            `  if(s==='System'||s==='system'){eff=window.matchMedia('(prefers-color-scheme: dark)').matches?'dark':'light'}`,
             `  else if(typeof s==='string'){eff=s.endsWith('Dark')||s==='dark'?'dark':(s.endsWith('Light')||s==='light'?'light':s)}`,
-            `  else{eff='light'}`,
+            `  else{eff='dark'}`,
             `  r.setAttribute('data-theme', (typeof s==='string'?s:eff));`,
             `  if(eff==='dark'){r.classList.add('dark'); r.classList.remove('light'); r.style.colorScheme='dark'}`,
             `  else if(eff==='light'){r.classList.add('light'); r.classList.remove('dark'); r.style.colorScheme='light'}`,