Add offline live spell checking (Hunspell via spellbook)

Spelling is now checked live as you type, fully offline, using the
pure-Rust `spellbook` crate to read Hunspell .aff/.dic dictionaries (no C
libhunspell — the binary stays self-contained; ldd unchanged).

- Canadian (en-CA, default) and British (en-GB) English dictionaries are
  compiled in; more languages are auto-discovered from system Hunspell
  folders and ~/.config/md-manuscript/dictionaries/. A "Spelling" row in
  the top bar toggles live checking and picks the dictionary.
- Misspellings are underlined in red; right-clicking a word opens a menu
  of suggested corrections. A results-panel and one-click fixes work too.
- Checks run on a background thread, debounced ~400ms after the last edit.
  Prose is extracted with pulldown-cmark so code spans, code blocks and
  link targets are skipped; ALL-CAPS initialisms are ignored.
- LanguageTool still layers on top: when its results are fresh they own
  the underlines (spelling + grammar); once you edit, the offline checker
  resumes. The editor underlines, issues panel and fix-apply logic are
  now shared between the two sources.

Bundled dictionaries are SCOWL-derived under a permissive license (kept
in dictionaries/<lang>/license). Adds spell-check unit tests (tokenizer,
code-block skipping, offset mapping, en-CA/en-GB spelling); 40 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N9kRuP7JvXoUGdNNeg5ZSs

Cargo.lock

@@ -1858,6 +1858,7 @@ dependencies = [
  "rfd",
  "serde",
  "serde_json",
+ "spellbook",
  "ureq",
  "zip",
 ]
@@ -3036,6 +3037,16 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "spellbook"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d204abcbdf8e88729306a8d0ca01a79d8a49969fe1f84696909d6dbc4321c1a"
+dependencies = [
+ "foldhash",
+ "hashbrown 0.17.1",
+]
+
 [[package]]
 name = "spirv"
 version = "0.3.0+sdk-1.3.268.0"