A11y + taxonomy UX batch

Motivation

Continuation of the UI/UX pass. Two more gaps from the deferred set are now code-verifiable without a live browser: toasts were not announced to assistive tech, and the taxonomy lineage tree could only be expanded one node at a time.

User-facing change

• #46 Toast live-region — the toast container is a labelled region, each toast carries role="alert"/aria-live="assertive" for errors and role="status"/aria-live="polite" otherwise, so screen readers announce notifications. The dismiss (✕) button gains an aria-label.
• #26 Taxonomy expand/collapse-all — the Lineage tree shows Expand all / Collapse all buttons. A bumped force signal is broadcast to every node; nodes react via useEffect and remain individually toggleable afterwards.

Code change summary

• web/src/components/Toast.tsx: region + per-toast role/aria-live, dismiss aria-label.
• web/src/pages/blastResults/analytics/TaxonomyPanel.tsx: forceSignal state in LineageTree, Expand/Collapse-all buttons, and a useEffect in LineageNodeView that applies the broadcast command.

Validation evidence

• cd web && npm run build → clean.
• cd web && npm test -- --run TaxonomyPanel → 5 passed.
• npx eslint on both changed files → clean.

No backend / API / IaC changes.