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/process-shim.mjs

@@ -0,0 +1,25 @@
+// Minimal `process` shim injected into the UI fixture bundle. Server-only
+// modules that leak into the bundle reference the `process` global at module
+// scope (release ids, paths, etc). esbuild's `inject` rewrites those free
+// identifier references to import from this file so evaluation cannot crash
+// in the browser.
+
+export const process = {
+	env: { NODE_ENV: "production" },
+	platform: "linux",
+	arch: "x64",
+	version: "v20.0.0",
+	versions: { node: "20.0.0" },
+	argv: [],
+	cwd: () => "/",
+	nextTick: (fn, ...args) => setTimeout(() => fn(...args), 0),
+	on: () => {},
+	once: () => {},
+	emit: () => {},
+	exit: () => {},
+	hrtime: Object.assign(() => [0, 0], { bigint: () => 0n }),
+	memoryUsage: () => ({ heapUsed: 0, rss: 0 }),
+	pid: 0,
+};
+
+export default process;