Default the Web App to :8090 (BFF) in the API Server

Point the API Server's Web App references at the production BFF port
instead of the Vite dev port (5173): the WEBAPP_URL default probed by
/api/status, and the panel's Web App URL / allowed-origins placeholders.
Update .env.example and the README env table to match. Rebuild the
embedded admin panel dist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API Server/panel/src/components/WebAppCard.vue

@@ -91,7 +91,7 @@ async function save() {
     <div class="flex flex-col gap-4 px-5 py-4">
       <div>
         <label class="dh-label" for="web-url">{{ t("webapp.baseUrl") }}</label>
-        <input id="web-url" v-model="form.url" class="dh-input" placeholder="http://localhost:5173" />
+        <input id="web-url" v-model="form.url" class="dh-input" placeholder="http://localhost:8090" />
       </div>
 
       <div>
@@ -100,7 +100,7 @@ async function save() {
           id="web-origins"
           v-model="form.origins"
           class="dh-input"
-          placeholder="http://localhost:5173, https://app.example.com"
+          placeholder="http://localhost:8090, https://app.example.com"
         />
         <p class="mt-1 text-xs text-muted">
           {{ tSplit("webapp.originsHint", "star").before