fix(menu): restructure main menu items for improved organization and clarity

Description

Related Issues

Closes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Enhancement (improvement to existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Security fix
• Dependency update
• CI/CD changes

Testing

Test Environment

• KOReader Version:
• Device(s) Tested:
• BookLore Plugin Version:

Test Cases

• Tested on actual e-reader device
• Tested with KOReader emulator
• Verified with unit tests
• Manual testing performed

Scenarios Tested

Checklist

• Code follows the project's coding style
• Self-review performed
• Comments added for complex/non-obvious code
• Documentation updated (if needed)
• CHANGELOG.md updated (if user-facing change)
• No new warnings introduced
• Tests added/updated (if applicable)
• All tests pass
• No breaking changes (or documented if unavoidable)
• Tested on physical device (for device-specific changes)

Screenshots/Logs

Migration Notes

Performance Impact

• No performance impact
• Performance improved
• Performance degraded (explain why acceptable):

Security Considerations

• No security impact
• Security improved
• Reviewed for security issues

Additional Notes

Summary by CodeRabbit

• Documentation
  • Added a “Main menu order” section and updated docs and README to use “Connection Setup” and new menu paths (Tools → BookLore Sync; Sync & Cache → Clear Cache/Pending).
• UI
  • Reorganized top-level BookLore Sync menu order and renamed “Shelf Downloads” to “Shelf Sync” with updated help text describing two‑way sync.
• Localization
  • Removed German “Shelf Downloads” entry and updated its description.
• Changelog
  • Added 4.0.0 release section and noted menu reorganization.