#355 serve: send Cache-Control: no-cache for index.html

Description

## Summary
- `/web/*` static assets (`app.js`, `app.css`, `sprite.svg`) already send `Cache-Control: no-cache`, but the root `index.html` response didn't set any cache header at all.
- Without it, the browser falls back to its own heuristic caching, so after an engine restart the page can keep showing a stale shell until the user hard-refreshes (Ctrl+F5) - even though `app.js`/`app.css` underneath were already being fetched fresh.
- One-line fix: give `index.html` the same `no-cache` treatment its own assets already have.

## Test plan
- [x] `python -m py_compile serve/server.py`
- [ ] Manual: start the server, load the page, restart the server, reload without a hard refresh - page should reflect the current `index.html` without needing Ctrl+F5.

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