feat: 双主题系统 — 编辑风 + 极简亮色

前端:
- 新增 Pinia theme store，localStorage 持久化，data-theme 切换
- App.vue: :root 新增 --font-*/--sidebar-* CSS 变量；新增 :root[data-theme=light] 完整配色（极简黑白金 + Noto Sans SC）
- DesktopLayout: 侧边栏 15 个变量化 + 顶栏主题切换按钮
- MobileLayout: 装饰色变量化
- Settings: 新增「界面主题」卡片
- 21 个组件 font-family 统一替换为 var(--font-*)
- 新增 utils/managerColor.ts — 12 色调色板 + 自适应文字色 + 后端自定义色优先
- WorkPlans/MiniBusiness/KeyVisits/CustomerManage: 客户经理标签色统一走共享工具
- UserManage: 编辑弹窗新增颜色选择器（仅 manager）
- LightBoard: 去标题星标图标
- ManagerWorkspace/WorkPlans/MiniBusiness/KeyVisits: 删除按钮改为实心红底白字

后端:
- User 模型新增 color 列 + lifespan 自动迁移
- users API 支持 color 字段读写
- UserOut schema 新增 color

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

frontend/src/stores/theme.ts

@@ -0,0 +1,30 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+export type Theme = 'editorial' | 'light'
+
+export const themeLabels: Record<Theme, string> = {
+  editorial: '编辑风',
+  light: '极简亮色',
+}
+
+export const useThemeStore = defineStore('theme', () => {
+  const currentTheme = ref<Theme>(
+    (localStorage.getItem('theme') as Theme) || 'editorial'
+  )
+
+  function applyTheme(theme: Theme) {
+    document.documentElement.setAttribute('data-theme', theme)
+  }
+
+  function setTheme(theme: Theme) {
+    currentTheme.value = theme
+    localStorage.setItem('theme', theme)
+    applyTheme(theme)
+  }
+
+  // Apply on init
+  applyTheme(currentTheme.value)
+
+  return { currentTheme, setTheme }
+})