fix(proofing): use macOS NSSpellChecker for spelling on macOS

The bundled Hunspell dictionary (~49k SCOWL roots) lacked common words like
"cybersecurity", "scalable", and "assistantship", and checked words inside
URLs. On macOS, spelling now uses the system NSSpellChecker (same engine as
TextEdit/Safari): far better vocabulary, skips URLs, and respects words the
user has taught macOS. The custom/tech dictionaries and Harper grammar are
unchanged; non-macOS builds keep the Hunspell fallback.

src-tauri/src/lib.rs

@@ -2,6 +2,8 @@ mod epub;
 mod fonts;
 mod gdrive;
 mod library;
+#[cfg(target_os = "macos")]
+mod macspell;
 mod pdf;
 mod project;
 mod proofing;