Fix login error feedback + load basecoat toast module

- inline error message on the login form (401 shows a visible French
  message, hidden again on submit)
- import basecoat-css/toast in BaseLayout: every showToast in the app
  was silently no-oping because the toast JS was never loaded

src/pages/api/login.ts

@@ -69,9 +69,12 @@ export const POST: APIRoute = async ({ request, cookies }) => {
         });
     } catch (err) {
         console.error("Login failed:", err);
-        return new Response(JSON.stringify({ error: "Invalid credentials" }), {
-            status: 401,
-            headers: { "Content-Type": "application/json" },
-        });
+        return new Response(
+            JSON.stringify({ error: "Email ou mot de passe incorrect" }),
+            {
+                status: 401,
+                headers: { "Content-Type": "application/json" },
+            },
+        );
     }
 };