feat(ui): unify link/address styles and set primary header color

- Links use primary color; removed leading icon
- Show API address when website is missing; use secondary text color, non-monospace
- Parse base_url from Codex TOML; change fallback copy to "未配置官网地址"
- Use primary color for the top-left header title
- Clean up unused imports

Affected files:
- src/components/ProviderList.tsx
- src/App.tsx

src/App.tsx

@@ -218,7 +218,7 @@ function App() {
       {/* Linear 风格的顶部导航 */}
       <header className="bg-white border-b border-[var(--color-border)] px-6 py-4">
         <div className="flex items-center justify-between">
-          <h1 className="text-xl font-semibold text-[var(--color-text-primary)]">
+          <h1 className="text-xl font-semibold text-[var(--color-primary)]">
             CC Switch
           </h1>