fix(ui): make fixture bundle browser-safe, fast and cacheable

The UI fixture bundle crashed at runtime, so every test waited out its
45s timeout:
- server-only modules leaked into the bundle and referenced node
  built-ins, the process global, and validated env at import time
- the previous platform: 'node' fix kept esbuild happy but emitted
  require() calls that cannot run in the browser

Fix it at the bundle boundary:
- alias node:* imports to e2e/ui/node-stubs.mjs (shape-compatible stubs,
  no real I/O)
- inject e2e/ui/process-shim.mjs for the process global
- define process.env.SKIP_ENV_VALIDATION=1 so src/env.ts skips parsing
  in the fixture (same escape hatch vitest uses)

Also make the fixture server fast to start:
- build JS and Tailwind CSS in parallel
- cache built assets in node_modules/.cache/nexst-ui-fixture keyed on a
  content hash of src/, fixtures, stubs and the server file
- startup drops from ~1.7s to ~220ms on cache hits

e2e/ui/node-stubs.mjs

@@ -1,6 +1,8 @@
 // Stubs for Node.js built-ins used by server-only modules that leak into the
 // UI fixture bundle. These keep esbuild from failing on `node:*` imports
 // while providing shape-compatible exports so the bundle stays loadable.
+// Nothing here performs real I/O; it only exists so module evaluation in the
+// browser does not crash on import of server-only helpers.
 
 export class AsyncLocalStorage {
 	enterWith() {}
@@ -13,24 +15,173 @@ export class AsyncLocalStorage {
 	}
 }
 
-export function createHash() {
-	return {
+const cryptoApi = {
+	createHash: () => ({
 		update: () => ({
 			digest: () => "",
 		}),
-	};
-}
+	}),
+	randomUUID: () => "00000000-0000-0000-0000-000000000000",
+	randomBytes: (size) => new Uint8Array(size),
+	createHmac: () => ({
+		update: () => ({
+			digest: () => "",
+		}),
+	}),
+	timingSafeEqual: () => true,
+	createCipheriv: () => ({
+		update: () => new Uint8Array(0),
+		final: () => new Uint8Array(0),
+	}),
+	createDecipheriv: () => ({
+		update: () => new Uint8Array(0),
+		final: () => new Uint8Array(0),
+	}),
+	webcrypto: globalThis.crypto,
+};
 
-export function randomUUID() {
-	return "00000000-0000-0000-0000-000000000000";
-}
+export const {
+	createHash,
+	randomUUID,
+	randomBytes,
+	createHmac,
+	timingSafeEqual,
+	createCipheriv,
+	createDecipheriv,
+	webcrypto,
+} = cryptoApi;
 
-export async function readFile() {
-	return Buffer.alloc(0);
-}
+export const appendFile = async () => {};
+export const mkdir = async () => {};
+export const readdir = async () => [];
+export const readFile = async () => new Uint8Array(0);
+export const rename = async () => {};
+export const stat = async () => ({
+	isDirectory: () => false,
+	isFile: () => true,
+	size: 0,
+	mtimeMs: 0,
+});
+export const unlink = async () => {};
+export const writeFile = async () => {};
+export const rm = async () => {};
+export const mkdtemp = async () => "/tmp/fixture-stub";
 
-export const path = {
+export const promises = {
+	appendFile,
+	mkdir,
+	readdir,
+	readFile,
+	rename,
+	stat,
+	unlink,
+	writeFile,
+	rm,
+	mkdtemp,
+};
+
+export const existsSync = () => false;
+export const readFileSync = () => new Uint8Array(0);
+export const writeFileSync = () => {};
+export const readdirSync = () => [];
+export const mkdirSync = () => {};
+export const rmSync = () => {};
+export const mkdtempSync = () => "/tmp/fixture-stub";
+export const appendFileSync = () => {};
+export const statSync = () => ({
+	isDirectory: () => false,
+	isFile: () => true,
+	size: 0,
+});
+
+const pathApi = {
 	join: (...parts) => parts.join("/"),
 	resolve: (...parts) => parts.join("/"),
+	dirname: (value) => value.split("/").slice(0, -1).join("/"),
+	basename: (value) => value.split("/").pop() ?? "",
+	extname: (value) => {
+		const base = value.split("/").pop() ?? "";
+		const index = base.lastIndexOf(".");
+		return index > 0 ? base.slice(index) : "";
+	},
+	relative: (_from, to) => to,
+	normalize: (value) => value,
+	isAbsolute: (value) => value.startsWith("/"),
 	sep: "/",
+	delimiter: ":",
+};
+
+export const {
+	join,
+	resolve,
+	dirname,
+	basename,
+	extname,
+	relative,
+	normalize,
+	isAbsolute,
+	sep,
+	delimiter,
+} = pathApi;
+
+export const path = pathApi;
+
+const osApi = {
+	hostname: () => "localhost",
+	tmpdir: () => "/tmp",
+	platform: () => "linux",
+	homedir: () => "/tmp",
+};
+
+export const { hostname, tmpdir, platform, homedir } = osApi;
+
+export const promisify = (fn) => fn;
+export const isIP = () => 0;
+export const lookup = async () => ({ address: "127.0.0.1", family: 4 });
+export const execFile = () => {};
+export const execFileSync = () => "";
+export const spawnSync = () => ({ status: 0, stdout: "", stderr: "" });
+export const fileURLToPath = (value) => String(value);
+export const createServer = () => ({
+	listen: () => {},
+	close: () => {},
+	on: () => {},
+	address: () => ({ port: 0 }),
+});
+
+// Single default export: all node:* aliases point at this one file, and the
+// merged object keeps `import crypto from "node:crypto"`, `import path from
+// "node:path"` and `import os from "node:os"` usable after aliasing.
+export default {
+	...cryptoApi,
+	...pathApi,
+	...osApi,
+	appendFile,
+	mkdir,
+	readdir,
+	readFile,
+	rename,
+	stat,
+	unlink,
+	writeFile,
+	rm,
+	mkdtemp,
+	promises,
+	existsSync,
+	readFileSync,
+	writeFileSync,
+	readdirSync,
+	mkdirSync,
+	rmSync,
+	mkdtempSync,
+	appendFileSync,
+	statSync,
+	promisify,
+	isIP,
+	lookup,
+	execFile,
+	execFileSync,
+	spawnSync,
+	fileURLToPath,
+	createServer,
 };