feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

frontend/app/ads/buy/AdsBuyClient.tsx

@@ -84,6 +84,9 @@ export default function AdsBuyClient() {
 
   const selectedPay: AdPaymentOption | undefined = cfg?.payments.find((p) => p.id === paymentID);
   const selectedDuration = cfg?.durations.find((d) => d.days === days);
+  const selectedPrice = selectedDuration
+    ? (kind === "text" ? selectedDuration.price_hint_text : selectedDuration.price_hint).trim()
+    : "";
   const canSubmit = !!cfg && cfg.payments.length > 0 && !busy;
   const kindLabel = kind === "image" ? "图片广告" : "文字广告";
 
@@ -224,6 +227,7 @@ export default function AdsBuyClient() {
           <p className="text-[12.5px]">
             {kindLabel}
             {selectedDuration ? ` · ${selectedDuration.label}` : ""}
+            {selectedPrice ? ` · ${selectedPrice}` : ""}
           </p>
         </div>
 
@@ -331,9 +335,17 @@ export default function AdsBuyClient() {
           <div className="grid grid-cols-5 gap-2">
             {cfg.durations.map((d) => {
               const active = days === d.days;
+              const price = (kind === "text" ? d.price_hint_text : d.price_hint).trim();
               return (
-                <ChipTab key={d.days} active={active} onClick={() => setDays(d.days)} compact>
-                  {d.label.replace(/\s/g, "")}
+                <ChipTab
+                  key={d.days}
+                  active={active}
+                  onClick={() => setDays(d.days)}
+                  compact
+                  priced={!!price}
+                >
+                  <span>{d.label.replace(/\s/g, "")}</span>
+                  {price ? <span className="j13-ads-chip-price">{price}</span> : null}
                 </ChipTab>
               );
             })}
@@ -455,17 +467,21 @@ function ChipTab({
   onClick,
   children,
   compact,
+  priced,
 }: {
   active: boolean;
   onClick: () => void;
   children: React.ReactNode;
   compact?: boolean;
+  priced?: boolean;
 }) {
   return (
     <button
       type="button"
       onClick={onClick}
-      className={`j13-ads-chip${active ? " is-on" : ""}${compact ? " is-compact" : ""}`}
+      className={`j13-ads-chip${active ? " is-on" : ""}${compact ? " is-compact" : ""}${
+        priced ? " is-priced" : ""
+      }`}
     >
       {children}
     </button>