feat(dictionary): add JSON import/export

[endlessovo99]

Summary

• add PersonalDictionaryStore import/export APIs for Typoless dictionary JSON files
• add import/export actions to the personal dictionary settings view
• document JSON dictionary import/export behavior and duplicate handling
• add store and view model tests for import/export, invalid JSON, legacy enabled migration, and duplicate skipping

Verification

• xcrun swiftc -typecheck -target arm64-apple-macosx14.0 app/Typoless/Persistence/PersonalDictionaryStore.swift app/Typoless/UI/Settings/PersonalDictionaryViewModel.swift
• git diff --check

Full xcodebuild tests were not run locally because this environment only has Command Line Tools selected and no Xcode.app/xcodegen installed.