refine: remove no-op reportWebVitals from playground

[prql-bot]

Found during the nightly survey of web/playground/src/.

reportWebVitals() is called with no argument in main.jsx, and its body is guarded on if (onPerfEntry && onPerfEntry instanceof Function) — so the guard is always false and the dynamic import("web-vitals") inside it never runs. The module is Create React App boilerplate (note the stale bit.ly/CRA-vitals link) that outlived the migration to Vite. This removes the module, its single call site, and the web-vitals dependency, which nothing else in the repo reaches.

The dependency is worth removing rather than leaving inert: Dependabot watches /web/playground for npm, so web-vitals generates PRs that a maintainer reviews and merges for code that cannot execute.

Verification

• web-vitals is referenced nowhere else — repo-wide grep over *.js, *.jsx, *.json, *.yaml, *.md (excluding node_modules) returns only reportWebVitals.js, the main.jsx import/call, and the package.json entry, all of which this PR removes.
• package-lock.json was edited surgically rather than regenerated: the runner's npm (10.9.8) is older than the one that wrote the lockfile and strips libc metadata from ~10 optional-dependency entries, which would be unrelated churn. The edit was cross-checked by running npm install --package-lock-only --ignore-scripts on a copy and diffing the resulting packages key set against the hand-edited file — the sets are identical, so the removal matches npm's own resolution.
• prettier --check passes on CHANGELOG.md, main.jsx, and package.json. Prettier fully parses the JSX, so the edited main.jsx is syntactically valid.

No regression test: the change deletes code that provably never executed, so there is no behavior to pin. The playground build in CI is the check that the removed import broke nothing.