fix: show detailed error messages for session creation failures

Return full error stack traces from the sessions API and display
the actual server error in the frontend, instead of a generic
"Failed to create session" message.

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

src/app/api/chat/sessions/route.ts

@@ -8,7 +8,8 @@ export async function GET() {
     const response: SessionsResponse = { sessions };
     return Response.json(response);
   } catch (error) {
-    const message = error instanceof Error ? error.message : 'Failed to fetch sessions';
+    const message = error instanceof Error ? error.stack || error.message : String(error);
+    console.error('[GET /api/chat/sessions] Error:', message);
     return Response.json({ error: message }, { status: 500 });
   }
 }
@@ -26,7 +27,8 @@ export async function POST(request: NextRequest) {
     const response: SessionResponse = { session };
     return Response.json(response, { status: 201 });
   } catch (error) {
-    const message = error instanceof Error ? error.message : 'Failed to create session';
+    const message = error instanceof Error ? error.stack || error.message : String(error);
+    console.error('[POST /api/chat/sessions] Error:', message);
     return Response.json({ error: message }, { status: 500 });
   }
 }

src/app/chat/page.tsx

@@ -111,7 +111,10 @@ export default function NewChatPage() {
           body: JSON.stringify(createBody),
         });
 
-        if (!createRes.ok) throw new Error('Failed to create session');
+        if (!createRes.ok) {
+          const errBody = await createRes.json().catch(() => ({}));
+          throw new Error(errBody.error || `Failed to create session (${createRes.status})`);
+        }
 
         const { session }: SessionResponse = await createRes.json();
         sessionId = session.id;