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/components/Editor/SuggestionHighlight.ts

@@ -22,7 +22,7 @@ interface PluginState {
 // mapOffset converts a character offset within a textblock's flattened text into
 // an absolute ProseMirror position, accounting for inline atoms (e.g. hard
 // breaks) that occupy a position but contribute no text.
-function mapOffset(block: PMNode, blockPos: number, targetOffset: number): number {
+export function mapOffset(block: PMNode, blockPos: number, targetOffset: number): number {
   let textOffset = 0
   let pmPos = blockPos + 1 // inline content starts just inside the block
   let result = pmPos