Integrate block-text editor into WordAI and adapt documents/tests

[Copilot]

WordAI editor needed richer block-based editing similar to react-block-text.

• Block editor integration

  • Replaced textarea canvas with react-block-text (header, list, quote, todo plugins) and consistent colors.
  • Added conversion utilities to wrap existing plain text into block-text JSON, extract plain text, ensure compatibility for loads/saves, and handle AI replacement.
  • Updated document lifecycle (new/load/restore/export/AI context) to work with block-based content.

• Test and runtime stability

  • Added React internals shim and jsdom-safe setup; mocked react-block-text in Vitest to avoid CSS/DOM incompatibilities.
  • Updated EditorCanvas tests to target the new editable surface; added blockText utility tests.
  • Downgraded React/ReactDOM to 18 for library compatibility and refreshed lockfile.

• UI/UX adjustments

  • Version history previews now show plain text from block values.
  • Editor key bindings preserved (AI trigger, export, version history, save).

Example conversion helper:

import { ensureBlockValue, extractPlainText } from '../utils/blockText';

const stored = ensureBlockValue(doc.content);
const readable = extractPlainText(stored); // for exports, AI context, previews