feat: add ads/sponsor/captcha modules and polish home & chat UI

frontend/lib/ads.ts

@@ -0,0 +1,8 @@
+/** 首页侧栏图片广告展示尺寸（与 .j13-ad-promo 宽一致：248 = 两列文字 + 中间 6px 间距） */
+export const AD_IMAGE_WIDTH = 248;
+export const AD_IMAGE_HEIGHT = 60;
+/** 单列文字槽宽度：(248 - 6) / 2 */
+export const AD_TEXT_COL_WIDTH = 121;
+/** 购买页建议文案 */
+export const AD_IMAGE_SIZE_LABEL = `${AD_IMAGE_WIDTH}×${AD_IMAGE_HEIGHT}`;
+export const AD_IMAGE_SIZE_LABEL_2X = `${AD_IMAGE_WIDTH * 2}×${AD_IMAGE_HEIGHT * 2}`;