feat(biochem): enlarge compound structure on hover or focus in reaction equation

[VibhavSetlur]

Summary

Sam noted on /biochem/reactions/rxn02005 that the atom mapping looks great but larger
compounds are hard to read at thumbnail size. This adds an enlarged structure preview:
hovering or keyboard-focusing a compound in the reaction equation pops up a 360px
render of that same structure, reusing the existing MoleculeRenderer and the exact same
atom-mapping colours as the thumbnail.

Behaviour

• Opens on mouseenter / focus, closes on mouseleave / blur / Escape.
• Uses MUI Popper (placement="top", pointerEvents: 'none') so the popup can never
  steal the pointer and cause hover flicker, and never traps focus.
• Preview receives the same smiles, atomColors, bondColors, showAllAtomLabels and
  fallbackSvg as the inline thumbnail, so atom-mapping colours are identical — just bigger.
• Deliberately does not pass onInventory / onGraph to the preview renderer, so the
  parent's colour-plan state is untouched by opening a preview.
• No preview for compounds with no drawn structure (e.g. H+ / cpd00067).
• role="tooltip" + aria-label="Enlarged structure of <name>", wired to the compound link
  via aria-describedby while open.
• Card is maxWidth: 90vw / maxHeight: 90vh with overflow: hidden so it stays contained
  on small viewports.

Tests

Four new focused tests in tests/unit/components/ReactionStructureEquation.test.tsx:
closed by default + opens on pointer enter (asserts 360px size, no onInventory/onGraph,
aria-describedby lifecycle, compound link href unchanged); opens on focus / closes on blur /
closes on Escape; preview reuses the thumbnail's atom-mapping colours (deep equality);
no preview for a compound without a drawn structure.

Verification (CI mirror, all exit 0)

npm run lint · npx tsc --noEmit · npm run test:run (33 files, 351 passed / 4 skipped) ·
npm run build · npm audit --omit=dev --audit-level=high (0 vulnerabilities) · git diff --check

Notes

• Branched from upstream/staging (4a11bb0) so it is independent of the still-open
  Solr-9 stoichiometry PR fix(biochem): support Solr 9 nested stoichiometry children #233.
• feat → minor bump 3.4.0 → 3.5.0 across VERSION.md, package.json, package-lock.json,
  CHANGELOG.md, per CONTRIBUTING.md. If fix(biochem): support Solr 9 nested stoichiometry children #233 merges first this will need a trivial
  version/changelog rebase.

[VibhavSetlur]

Updated against current `staging` (825fd56), which now contains #235 (shared Solr base URL) and #233 (Solr 9 nested stoichiometry).

3216782 — `git merge --no-ff upstream/staging`. Conflicts were release metadata only (`VERSION.md`, `package.json`, `package-lock.json`, `CHANGELOG.md`); resolved to 3.5.0 over the released 3.4.1 (feat → minor, per CONTRIBUTING), with `[3.5.0]` inserted above the unaltered `[3.4.1]` and `[Unreleased]` sections. No dependency drift in the lockfile (only the two version strings). No source or Solr-config conflicts — @samseaver's e788d98 comes through from staging byte-identical and appears exactly once.

9902ede — 3-line test fix in `tests/unit/api/biochemStoichiometry.test.ts` (93/98/101). This is a pre-existing semantic conflict on `staging`, not something this branch introduced: #233 branched from 4a11bb0 and merged after #235 without containing it, so its expected URLs still named `staging.modelseed.org/solr`. Confirmed failing on a pristine `staging` checkout. Only the host changed; paths, query strings, encoding, and the `reactions_staging` collection name are byte-identical, since the shared instance serves both core sets.

Full CI mirror green locally and in Verify: lint, tsc --noEmit, test:run (358 passed / 4 skipped), build, audit (0 vulns).