feat: implement next redirect flow, remove inactivity auto-logout, and add concurrent session termination notification

shop/components/login-form.tsx

@@ -90,6 +90,17 @@ export function LoginForm({
                   onChange={(e) => setPassword(e.target.value)}
                 />
               </div>
+            {(() => {
+              const messageParam = typeof window !== 'undefined' ? new URLSearchParams(window.location.search).get('message') : null;
+              if (messageParam === "concurrent") {
+                return (
+                  <p className="text-sm text-amber-500 bg-amber-500/10 border border-amber-500/20 p-3 rounded-lg text-center font-medium">
+                    Sie wurden abgemeldet, da Sie sich an einem anderen Gerät angemeldet haben.
+                  </p>
+                );
+              }
+              return null;
+            })()}
             {error && <p className="text-sm text-red-500">{error}</p>}
             <Button type="submit" className="w-full" disabled={isLoading}>
               {isLoading ? "Logging in..." : "Login"}