Add basic login screen with form-based authentication

Adds /login route with HTML template that handles GET (show form) and
POST (authenticate). On successful login, sets session cookie and
redirects to /. Also adds framework support for redirects and cookies
in route handlers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

express/types.ts

@@ -49,12 +49,35 @@ export type ProcessedRoute = {
     handler: InternalHandler;
 };
 
+export type CookieOptions = {
+    httpOnly?: boolean;
+    secure?: boolean;
+    sameSite?: "strict" | "lax" | "none";
+    maxAge?: number;
+    path?: string;
+};
+
+export type Cookie = {
+    name: string;
+    value: string;
+    options?: CookieOptions;
+};
+
 export type Result = {
     code: HttpCode;
     contentType: ContentType;
     result: string;
+    cookies?: Cookie[];
 };
 
+export type RedirectResult = Result & {
+    redirect: string;
+};
+
+export function isRedirect(result: Result): result is RedirectResult {
+    return "redirect" in result;
+}
+
 export type Route = {
     path: string;
     methods: Method[];