feat(auth): add return-URL flow for login redirects

Add LoginLink client component that captures current path via usePathname()
and passes it as returnTo query param. Update login page to read and
sanitize returnTo (open-redirect guard: must start with /, reject //).
Update LoginForm to redirect to returnTo after successful login. Wire
LandingPage CTA through LoginLink. Add returnTo redirect for already-
authed users hitting /login. Add return-URL to flappy page redirect.
Remove trailing blank line from (frontend) layout.

src/app/(frontend)/flappy/page.tsx

@@ -18,7 +18,8 @@ export default async function FlappyPage() {
   const { user } = await payload.auth({ headers, canSetHeaders: false });
 
   if (!user) {
-    redirect("/login");
+    const pathname = headers.get("x-invoke-path") || "/flappy";
+    redirect(`/login?returnTo=${encodeURIComponent(pathname)}`);
   }
 
   const profileRes = await payload.find({

src/app/(frontend)/layout.tsx

@@ -40,7 +40,6 @@ export const metadata: Metadata = {
 
 export default async function RootLayout(props: { children: React.ReactNode }) {
   const { children } = props;
-
   const headers = await nextHeaders();
 
   const payloadConfig = await config;

src/app/login/page.tsx

@@ -10,12 +10,24 @@ import AppLogo from "@/components/graphics/AppLogo";
 // (no DB is available at build time in a container).
 export const dynamic = "force-dynamic";
 
-export default async function LoginPage() {
+function safeReturnTo(value: string | undefined): string | undefined {
+  if (!value?.startsWith("/") || value.startsWith("//")) return undefined;
+  return value;
+}
+
+export default async function LoginPage({
+  searchParams,
+}: {
+  searchParams: Promise<{ returnTo?: string }>;
+}) {
+  const { returnTo } = await searchParams;
+  const target = safeReturnTo(returnTo);
+
   const payload = await getPayload({ config });
   const headers = await nextHeaders();
   const { user } = await payload.auth({ headers, canSetHeaders: false });
 
-  if (user) redirect("/");
+  if (user) redirect(target ?? "/");
 
   return (
     <div className="min-h-screen flex flex-col items-center justify-center p-5 gap-6 bg-background">
@@ -27,7 +39,7 @@ export default async function LoginPage() {
           Log in to access the operations dashboard, intelligence, and logistics.
         </p>
       </div>
-      <LoginForm />
+      <LoginForm returnTo={target} />
     </div>
   );
 }

src/components/frontend/LandingPage.tsx

@@ -1,6 +1,6 @@
-import Link from "next/link";
 import { Button } from "@/components/ui/button";
 import AppLogo from "@/components/graphics/AppLogo";
+import { LoginLink } from "@/components/frontend/auth/LoginLink";
 
 export function LandingPage() {
   return (
@@ -14,7 +14,7 @@ export function LandingPage() {
           operations dashboard, intelligence, and logistics.
         </p>
         <Button asChild className="mt-2">
-          <Link href="/login">Log in to access the portal</Link>
+          <LoginLink>Log in to access the portal</LoginLink>
         </Button>
       </div>
     </div>

src/components/frontend/auth/LoginForm.tsx

@@ -7,7 +7,11 @@ import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 
-export function LoginForm() {
+interface LoginFormProps {
+  returnTo?: string;
+}
+
+export function LoginForm({ returnTo }: LoginFormProps) {
   const router = useRouter();
   const [username, setUsername] = useState("");
   const [password, setPassword] = useState("");
@@ -33,7 +37,12 @@ export function LoginForm() {
         return;
       }
 
-      router.push("/");
+      // Redirect to returnTo if provided, otherwise to home
+      if (returnTo && returnTo !== "/") {
+        router.push(returnTo);
+      } else {
+        router.push("/");
+      }
       router.refresh();
     } catch {
       setError("Something went wrong. Please try again.");

src/components/frontend/auth/LoginLink.tsx

@@ -0,0 +1,12 @@
+"use client";
+
+import Link from "next/link";
+import { usePathname } from "next/navigation";
+import type { ComponentProps } from "react";
+
+type LoginLinkProps = Omit<ComponentProps<typeof Link>, "href">;
+
+export function LoginLink(props: LoginLinkProps) {
+  const pathname = usePathname();
+  return <Link {...props} href={`/login?returnTo=${encodeURIComponent(pathname)}`} />;
+}