前端: 修复浅色模式白字 + 主题优化

- 修 nav 激活态残留 text-zinc-50 导致浅色下白字白底
- @theme 令牌改实色字面量, 浅色用 [data-theme=light] 覆盖同一批 --color-*
- 新增 soft 令牌(accent/ok/warn/err, color-mix 运行时解析)替代烘焙的透明度修饰符,
  修复 badge 实色底、focus ring 实线等问题
- ThemeToggle 改三选下拉(浅色/深色/跟随系统), 系统自动为显式选项
- 浅色强调色调深保证 WCAG 对比度, 全量复查无残留硬编码颜色类

Co-Authored-By: Claude <noreply@anthropic.com>

web/src/views/admin/ConfigView.vue

@@ -53,14 +53,14 @@ onMounted(load)
             <div class="flex gap-2">
               <button
                 class="flex-1 rounded-md border px-3 py-2 text-sm transition"
-                :class="config.registration_mode === 'open' ? 'border-accent bg-accent/10 text-accent' : 'border-edge2 text-muted'"
+                :class="config.registration_mode === 'open' ? 'border-accent bg-accent-soft text-accent' : 'border-edge2 text-muted'"
                 @click="config.registration_mode = 'open'"
               >
                 开放注册
               </button>
               <button
                 class="flex-1 rounded-md border px-3 py-2 text-sm transition"
-                :class="config.registration_mode === 'invite' ? 'border-accent bg-accent/10 text-accent' : 'border-edge2 text-muted'"
+                :class="config.registration_mode === 'invite' ? 'border-accent bg-accent-soft text-accent' : 'border-edge2 text-muted'"
                 @click="config.registration_mode = 'invite'"
               >
                 邀请码
@@ -78,7 +78,7 @@ onMounted(load)
             />
           </div>
 
-          <div class="flex items-center justify-between rounded-md border border-edge bg-surface/60 px-4 py-3">
+          <div class="flex items-center justify-between rounded-md border border-edge bg-surface px-4 py-3">
             <div>
               <p class="text-sm text-ink">其他配置项</p>
               <p class="text-xs text-muted">汇率、限流阈值、维护开关在后续里程碑开放</p>