refactor(styles): 建立统一的生产级样式体系 (#4)

## 变更内容

- 将全局样式拆分为设计令牌、基础规则和语义组件层
- 集中管理 Naive UI 主题配置
- 迁移页面容器、表单、信息面板和按钮等重复样式
- 增加样式体系文档与自动契约检查

## 验证

- `npm run check:styles`
- `npm run build`Reviewed-on: #4

src/lib/theme.ts

@@ -0,0 +1,55 @@
+import type { GlobalThemeOverrides } from 'naive-ui'
+
+/** Keep these values aligned with the semantic tokens in styles/tokens.css. */
+export const openCloudThemeOverrides: GlobalThemeOverrides = {
+  common: {
+    primaryColor: '#0f766e',
+    primaryColorHover: '#115e59',
+    primaryColorPressed: '#134e4a',
+    primaryColorSuppl: '#0f766e',
+    infoColor: '#0369a1',
+    successColor: '#0f766e',
+    warningColor: '#d97706',
+    errorColor: '#dc2626',
+    borderRadius: '0px',
+    borderRadiusSmall: '0px',
+    fontFamily: '"IBM Plex Sans", "Noto Sans SC", "PingFang SC", sans-serif',
+    fontFamilyMono: '"IBM Plex Mono", "SFMono-Regular", monospace',
+    bodyColor: '#f3f7fb',
+    cardColor: '#ffffff',
+    modalColor: '#ffffff',
+    popoverColor: '#ffffff',
+    tableColor: '#ffffff',
+    textColorBase: '#0f172a',
+    textColor1: '#0f172a',
+    textColor2: '#334155',
+    textColor3: '#64748b',
+    textColorDisabled: '#94a3b8',
+    dividerColor: '#d9e3ee',
+    borderColor: '#d9e3ee',
+    inputColor: '#ffffff',
+  },
+  Button: {
+    borderRadiusTiny: '0px',
+    borderRadiusSmall: '0px',
+    borderRadiusMedium: '0px',
+    borderRadiusLarge: '0px',
+    textColorPrimary: '#0f172a',
+    textColorHoverPrimary: '#0f172a',
+  },
+  Card: {
+    borderRadius: '0px',
+    borderRadiusEmbedded: '0px',
+    paddingMedium: '24px',
+  },
+  Input: { borderRadius: '0px' },
+  Select: {
+    peers: {
+      InternalSelection: { borderRadius: '0px' },
+    },
+  },
+  Tabs: { tabBorderRadius: '0px' },
+  Tag: { borderRadius: '0px' },
+  Alert: { borderRadius: '0px' },
+  Modal: { borderRadius: '0px' },
+}