docs: fix broken dashboard image on i18n pages (#38031)

The dashboard screenshot uses a relative path `src="whatsapp-openclaw.jpg"`
which resolves correctly on the English root page but produces 404 on
zh-CN and ja-JP pages because Mintlify prepends the language subdirectory
to the CDN path.

Change to absolute path `/whatsapp-openclaw.jpg` in all three index files,
consistent with other images on the same page that already use absolute
paths (e.g. `/assets/openclaw-logo-text-dark.png`).

docs/index.md

@@ -124,7 +124,7 @@ Open the browser Control UI after the Gateway starts.
 - Remote access: [Web surfaces](/web) and [Tailscale](/gateway/tailscale)
 
 <p align="center">
-  <img src="whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
+  <img src="/whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
 </p>
 
 ## Configuration (optional)

docs/ja-JP/index.md

@@ -118,7 +118,7 @@ Gatewayの起動後、ブラウザでControl UIを開きます。
 - リモートアクセス: [Webサーフェス](/web)および[Tailscale](/gateway/tailscale)
 
 <p align="center">
-  <img src="whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
+  <img src="/whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
 </p>
 
 ## 設定（オプション）

docs/zh-CN/index.md

@@ -118,7 +118,7 @@ Gateway 网关启动后，打开浏览器控制界面。
 - 远程访问：[Web 界面](/web)和 [Tailscale](/gateway/tailscale)
 
 <p align="center">
-  <img src="whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
+  <img src="/whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
 </p>
 
 ## 配置（可选）