Phase 7: browser-side spell check (nspell, en-US)

Vendored Hunspell en.aff/en.dic into web/public/dictionaries/en/ (served as a
static asset + embedded in the binary, kept out of the JS bundle). dictionary-en
moved to a devDep — only used to source the files.

- useSpellChecker (App-level, loads once/session): fetches the dict, builds an
  nspell instance, replays a localStorage personal word list; addWord persists
  and bumps a version so consumers re-decorate. Ambient types in
  src/types/nspell.d.ts (the package ships none).
- SpellCheck Tiptap extension: misspellings as ProseMirror decorations (no
  stored marks), recomputed on edit / caret move / checker swap. Latin-only
  tokenizer so CJK is never flagged; skips short tokens + all-caps acronyms;
  exempts the caret word to avoid mid-typing jitter. Reuses mapOffset (now
  exported from SuggestionHighlight); wordAt resolves the exact span on click.
- MisspellCard: soft rose wavy underline, bilingual popover with up to 5 nspell
  corrections (click to replace) + add-to-dictionary. Closes on outside-pointer,
  edit, or doc switch.

Chinese spell check intentionally omitted — nspell is dictionary-based and
English-only; Chinese typos (homophone 别字) need an LLM, out of v1 scope.

tsc/vite/go build+vet clean; live server serves both dict files; nspell
behavior smoke-tested.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/types/nspell.d.ts

@@ -0,0 +1,19 @@
+// Minimal ambient types for nspell (the package ships no declarations). We use
+// only the handful of methods the spell checker needs: correctness lookup,
+// correction suggestions, and adding words to the in-memory personal dictionary.
+declare module 'nspell' {
+  export interface NSpell {
+    correct(word: string): boolean
+    suggest(word: string): string[]
+    add(word: string, model?: string): NSpell
+    remove(word: string): NSpell
+  }
+  export interface Dictionary {
+    aff: string | Buffer
+    dic: string | Buffer
+  }
+  export default function nspell(
+    aff: string | Buffer | Dictionary,
+    dic?: string | Buffer,
+  ): NSpell
+}