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/util.ts

@@ -3,7 +3,7 @@ import nunjucks from "nunjucks";
 import { contentTypes } from "./content-types";
 import { executionContext } from "./execution-context";
 import { httpCodes } from "./http-codes";
-import type { Result } from "./types";
+import type { Result, RedirectResult } from "./types";
 
 // FIXME: Handle the error here
 const loadFile = async (path: string): Promise<string> => {
@@ -13,12 +13,14 @@ const loadFile = async (path: string): Promise<string> => {
     return data;
 };
 
-const render = async (path: string, ctx: object): Promise<string> => {
+const render = async (path: string, ctx?: object): Promise<string> => {
     const fullPath = `${executionContext.diachron_root}/templates/${path}.html.njk`;
 
     const template = await loadFile(fullPath);
 
-    const retval = nunjucks.renderString(template, ctx);
+    const c = ctx===undefined ? {} : ctx;
+
+    const retval = nunjucks.renderString(template, c);
 
     return retval;
 };
@@ -33,4 +35,13 @@ const html = (payload: string): Result => {
     return retval;
 };
 
-export { render, html };
+const redirect = (location: string): RedirectResult => {
+    return {
+        code: httpCodes.redirection.SeeOther,
+        contentType: contentTypes.text.plain,
+        result: "",
+        redirect: location,
+    };
+};
+
+export { render, html, redirect };