feat(editor): add spell & grammar check engines with decorations

Wire real spell-checking into the editor via hunspell-wasm (bundled en_US
dictionary) and an optional LanguageTool grammar client.

- spell/: HunspellSpellEngine wrapper, pure spell logic, base64 en_US dict
- grammar/: LanguageToolEngine grammar client and pure grammar logic
- decorations/: collectBlock text assembly, offset-mapped CheckService,
  buildCheckDecorations (flat DecorationSet, inline spell/grammar marks)
- export engines and types from editor index; add hunspell-wasm dep

Verified: typecheck (6 pkgs), tests (all pass), lint (6 pkgs), and web
build all green.

pnpm-lock.yaml

@@ -133,6 +133,9 @@ importers:
       '@tiptap/starter-kit':
         specifier: 3.32.0
         version: 3.32.0
+      hunspell-wasm:
+        specifier: ^0.3.0
+        version: 0.3.0
     devDependencies:
       '@types/node':
         specifier: ^22.10.2
@@ -2110,6 +2113,9 @@ packages:
     resolution: {integrity: sha512-vK9P5/iUfdl95AI+JVyUuIcVtd4ofvtrOr3HNtM2yxC9bnMbEdp3x01OhQNnjb8IJYi38VlTE3mBXwcfvywuSw==}
     engines: {node: '>= 14'}
 
+  hunspell-wasm@0.3.0:
+    resolution: {integrity: sha512-D7/QLaeekJYYlE37D7KBq5v38vL4tQutTFIUBOUGBl0g9la8OUgfSd8+XbwNPV5uYMHS13umXR9MtdAxStpHJg==}
+
   iconv-lite@0.6.3:
     resolution: {integrity: sha512-4fCk79wshMdzMp2rH06qWrJE4iolqLhCUH+OiuIgU++RB0+94NlDL81atO7GX55uUKueo0txHNtvEyI6D7WdMw==}
     engines: {node: '>=0.10.0'}
@@ -4875,6 +4881,8 @@ snapshots:
     transitivePeerDependencies:
       - supports-color
 
+  hunspell-wasm@0.3.0: {}
+
   iconv-lite@0.6.3:
     dependencies:
       safer-buffer: 2.1.2