Improve grid keyboard selection visibility

[cloudycotton]

Closes #376.

What changed

• Update selected table arguments from SlickGrid's onActiveCellChanged event, so arrow-key navigation stays in sync with mouse selection.
• Keep existing Click action handlers mouse-only to avoid triggering side effects while a user navigates with the keyboard.
• Add a clear active-row highlight and stronger active-cell focus treatment in both light and dark themes.
• Guard transient invalid row/column positions and cover the selection adapter with focused tests.

Verification

• CI=true yarn test --watchAll=false --runInBand AGridSelection
• yarn eslint src/components/AGridSelection.ts src/components/AGridSelection.test.ts src/components/AGrid.tsx

The repository's committed lockfiles are currently out of sync with package.json, so a clean full build is blocked by existing dependency/type drift unrelated to this patch.

[cloudycotton]

@ryanhamilton Implementation and interactive proof are ready: https://cloudycotton.github.io/local-service-sites/previews/pulse-grid-selection/

Focused tests and ESLint checks pass. I also sent the accompanying fixed-price proposal to react@timestored.com: $300 for this merge-ready change, focused tests, and one review round, invoiced only after acceptance. If TimeStored accepts those terms, please reply here or by email with Approved: $300 paid trial. Until then, I am treating this as an unaccepted proposal and not as paid work.