fix: normalize API URL to prevent duplicate /chat/completions path

Users entering the full endpoint URL (e.g. https://openrouter.ai/api/v1/chat/completions)
caused a 404 because the code appended /chat/completions again. Now strips any trailing
/chat/completions before re-appending, and adds a hint in the UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/common/utils/ai-caller.ts

@@ -37,7 +37,12 @@ export async function callOpenAICompatible(params: AICallerParams): Promise<AICa
   const https = await import('https');
 
   const aiResult = await new Promise<{ status: number; body: string }>((resolve, reject) => {
-    const url = new URL(`${apiUrl}/chat/completions`);
+    // Normalize: strip trailing slash and /chat/completions if user included it
+    let baseUrl = apiUrl.replace(/\/+$/, '');
+    if (baseUrl.endsWith('/chat/completions')) {
+      baseUrl = baseUrl.slice(0, -'/chat/completions'.length);
+    }
+    const url = new URL(`${baseUrl}/chat/completions`);
 
     const options = {
       hostname: url.hostname,