Replace the in-vault ui.config.md file with localStorage persistence
keyed by vault path. Vaults now contain only user content — no app
config files. Removes the Rust vault_config module, Tauri commands
(get/save_vault_config), startup migrations, and demo vault config files.
The reactive store (vaultConfigStore) and consumer hooks (useZoom,
useViewMode, useRawMode) are unchanged — only the storage backend moved.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>