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.

docker/languagetool/compose.yaml

@@ -0,0 +1,24 @@
+# LanguageTool server for grammar checking.
+#
+# Usage:
+#   docker compose -f docker/compose.yaml -f docker/languagetool/compose.yaml up
+#
+# The LanguageTool server listens on port 8010 by default.
+# Set `grammarProvider: "languagetool"` and `grammarServerUrl: "http://localhost:8010"`
+# in PubScript settings to enable network-based grammar checking.
+
+services:
+  languagetool:
+    image: erikvl87/languagetool-server:latest
+    ports:
+      - "8010:8010"
+    environment:
+      - LANGUAGETOOL_HOME=/home/languagetool
+    volumes:
+      - ./languagetool-data:/home/languagetool
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "wget", "--spider", "http://localhost:8010/v2/check"]
+      interval: 10s
+      timeout: 5s
+      retries: 5