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

src/config.rs

@@ -45,6 +45,24 @@ pub struct Config {
     /// Language passed to LanguageTool (`auto` to detect, or a code like `en-US`).
     #[serde(default = "default_languagetool_language")]
     pub languagetool_language: String,
+    /// Whether the offline (Hunspell) live spell checker underlines misspellings
+    /// as you type. Used whenever LanguageTool results aren't current.
+    #[serde(default = "default_spell_check")]
+    pub spell_check: bool,
+    /// Id of the spell-check dictionary to use (e.g. `en-CA`, `en_GB`), matching
+    /// a [`crate::spell::DictEntry::id`].
+    #[serde(default = "default_spell_language")]
+    pub spell_language: String,
+}
+
+/// Live spell checking is on by default.
+pub fn default_spell_check() -> bool {
+    true
+}
+
+/// Default dictionary: the built-in Canadian English one.
+pub fn default_spell_language() -> String {
+    crate::spell::DEFAULT_LANGUAGE.to_string()
 }
 
 /// Default scheme: plain HTTP, matching a local server.
@@ -89,6 +107,8 @@ impl Default for Config {
             languagetool_port: default_languagetool_port(),
             languagetool_token: String::new(),
             languagetool_language: default_languagetool_language(),
+            spell_check: default_spell_check(),
+            spell_language: default_spell_language(),
         }
     }
 }