style(providers): restore the light-blue "Get API Key" link

The v7 pass turned the link into underlined body text, which read like a
plain label. Bring back the pre-v7 blue (blue-400 light / blue-500 dark,
swapped on hover) and drop the underline; keep the caption size and the
↗ arrow.

src/components/providers/forms/shared/ApiKeySection.tsx

@@ -81,7 +81,7 @@ export function ApiKeySection({
             href={websiteUrl}
             target="_blank"
             rel="noopener noreferrer"
-            className="text-caption text-fg-1 underline underline-offset-2 hover:text-fg-2"
+            className="text-caption text-blue-400 dark:text-blue-500 hover:text-blue-500 dark:hover:text-blue-400 transition-colors"
           >
             {t("providerForm.getApiKey", { defaultValue: "获取 API Key" })} ↗
           </a>