feat(editor): spell & grammar checking (Chunk #7)

Offline spellcheck via hunspell compiled to WASM (runs fully in-browser),
plus optional self-hostable LanguageTool grammar/style checking.

- hunspell-wasm spell engine with bundled en_US dictionary (base64-decoded
  browser-safe via atob + TextDecoder)
- LanguageTool engine (self-hostable, docker compose snippet under
  docker/languagetool/)
- single decoration pipeline: spell + grammar hits merge into one squiggle
  layer (red=spelling, blue=grammar) with a side panel to jump/apply/ignore
  and add-to-dictionary
- useCheckService hook wired into WritingView; check.css loaded in main.tsx
- spell + grammar unit tests (30 editor tests green)

PLAN.md and CHUNK#7.md updated to reflect completion.

packages/editor/src/decorations/index.ts

@@ -2,3 +2,4 @@ export { CheckService } from "./check-service.js";
 export type { CheckServiceOptions, CheckResult, CheckCounts } from "./check-service.js";
 export { buildCheckDecorations } from "./decorations.js";
 export type { CheckedIssue, BlockIssues } from "./decorations.js";
+export { toCheckedIssues, collectBlock, textOffsetToDocPos } from "./decorations.js";