feat: add preferences import and export

[zefutoff]

Summary

• add versioned JSON export/import for non-sensitive correction preferences
• export selected dictionary and Grammalecte rule overrides
• intentionally exclude the personal dictionary
• validate format, version, dictionary values and rule override types
• reject unknown rule IDs before modifying current settings
• persist imported preferences across engine restarts
• preserve the personal dictionary during imports
• use Android's system document picker for import/export
• require no storage permission
• mark non-sensitive preference import/export complete in the roadmap

Format

Current export format version: 1

Exported data:

• selected bundled dictionary
• Grammalecte rule overrides

Intentionally excluded:

• personal dictionary
• Android/device state
• identifiers
• usage data
• any other application data

Validation

• JSON round-trip tests
• invalid format/version/dictionary/rule-value tests
• unknown rule IDs rejected before state changes
• real QuickJS / Grammalecte import/export tests
• Android instrumentation verifies persistence across engine restart
• personal dictionary remains untouched
• manual export/import validation on Samsung Galaxy S21 / Android 15
• Android Storage Access Framework used without storage permissions
• make check
• make assemble
• git diff --check
• final APK does not request android.permission.INTERNET