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

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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

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