- New restore_default_themes() Rust fn: seeds both _themes/ and theme/ dirs
- Per-file idempotent: never overwrites existing files with content
- Fixed ensure_vault_themes() to include minimal.md (was missing)
- New 'Restore Default Themes' command in Cmd+K Appearance group
- 3 new Rust tests + 4 new frontend tests
- 1676 frontend tests passed