Add a local spelling dictionary

[augiefra]

Summary

Adds a local-only spelling dictionary and a native correction flow from the menu bar.

• Create an empty ~/.config/parrot/dictionary.json on first use.
• Apply deterministic replacements after Whisper transcription.
• Reload the file for every dictation, so a saved correction needs no daemon restart.
• Match whole Unicode words and preserve longer words.
• Add Add dictionary correction… in the menu bar for a guided native form.
• Keep all vocabulary and transcripts on the Mac; this does not retrain or call a remote model.

Validation

• swift test — 3 dictionary tests passed, including a literal replacement containing $.
• git diff --check passed.
• The default dictionary is empty and the documentation/tests use neutral example terms.
• No app bundle, signing identity, certificate, downloaded model, icon, personal dictionary, or user-specific vocabulary is included.

Coordination

This is functionally independent from #5, but both PRs add the shared Swift test target and touch the menu/README. If #5 lands first, I can rebase this PR to remove those small overlaps.

Contribution terms

The repository does not currently declare a LICENSE. I am offering these changes to the repository maintainer for review; a project license or contribution policy would make the intended redistribution terms explicit.