feat: Add 'Restore Default Themes' command for vaults missing theme structure

- 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

src/hooks/useAppCommands.ts

@@ -62,6 +62,7 @@ interface AppCommandsConfig {
   onCheckForUpdates?: () => void
   onRemoveActiveVault?: () => void
   onRestoreGettingStarted?: () => void
+  onRestoreDefaultThemes?: () => void
   isGettingStartedHidden?: boolean
   vaultCount?: number
   mcpStatus?: string
@@ -172,6 +173,7 @@ export function useAppCommands(config: AppCommandsConfig): CommandAction[] {
     onCheckForUpdates: config.onCheckForUpdates,
     onRemoveActiveVault: config.onRemoveActiveVault,
     onRestoreGettingStarted: config.onRestoreGettingStarted,
+    onRestoreDefaultThemes: config.onRestoreDefaultThemes,
     isGettingStartedHidden: config.isGettingStartedHidden,
     vaultCount: config.vaultCount,
     mcpStatus: config.mcpStatus,