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/components/ui/Button.vue

@@ -17,8 +17,8 @@ withDefaults(
     :class="[
       size === 'sm' ? 'h-8 px-3 text-xs' : 'h-10 px-4 text-sm',
       variant === 'primary' && 'bg-accent text-accent-ink hover:bg-accent-strong',
-      variant === 'ghost' && 'border border-edge2 text-ink hover:border-edge2 hover:bg-surface2/60',
-      variant === 'danger' && 'border border-err/50 text-err hover:border-err hover:bg-err/10',
+      variant === 'ghost' && 'border border-edge2 text-ink hover:border-edge2 hover:bg-surface2',
+      variant === 'danger' && 'border border-err text-err hover:border-err hover:bg-err-soft',
     ]"
   >
     <span v-if="loading" class="size-3.5 animate-spin rounded-full border-2 border-current border-t-transparent" />