perf(react-ui): code-split bundle, speed up coverage suite (#10042)

* Curate the highlight.js build to ~29 languages (lib/core + the
  common set) instead of the full ~190-grammar default: -787 KB raw /
  -230 KB gz on the base bundle.
* Code-split every route via React.lazy with a per-layout <Suspense>
  in App.jsx so the sidebar stays mounted on navigation. Initial entry
  chunk drops from 3194 KB raw / 887 KB gz to 397 KB / 122 KB (-87%).
  Warm chunks on sidebar hover/focus/touch via a preload registry so
  the click finds the chunk already in flight or cached.
* Migrate Playwright coverage from istanbul (build-time counters) to
  native Chromium V8 coverage, with per-worker accumulation +
  conversion. Suite drops from 71s to 30s at 20 workers (~58%) at the
  non-instrumented floor.
* Keep the coverage gate bundling-invariant: the coverage build inlines
  dynamic imports so every shipped source file lands in the denominator
  (otherwise untested page chunks silently drop out and inflate the
  percentage). Production builds stay code-split.
* Add UI_TEST_WORKERS=N Makefile knob; tighten coverage tolerance to
  0.8pp now that jitter sits near istanbul's ~0.5pp again.

Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Richard Palethorpe <io@richiejp.com>