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,

src/hooks/useCommandRegistry.test.ts

@@ -703,6 +703,41 @@ describe('useCommandRegistry', () => {
       expect(cmd!.keywords).toContain('demo')
     })
   })
+
+  describe('restore-default-themes command', () => {
+    it('has restore-default-themes command in Appearance group', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd).toBeDefined()
+      expect(cmd!.label).toBe('Restore Default Themes')
+      expect(cmd!.group).toBe('Appearance')
+      expect(cmd!.enabled).toBe(true)
+    })
+
+    it('is disabled when onRestoreDefaultThemes is not provided', () => {
+      const { result } = renderHook(() => useCommandRegistry(makeConfig()))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd).toBeDefined()
+      expect(cmd!.enabled).toBe(false)
+    })
+
+    it('calls onRestoreDefaultThemes when executed', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      result.current.find(c => c.id === 'restore-default-themes')!.execute()
+      expect(onRestoreDefaultThemes).toHaveBeenCalled()
+    })
+
+    it('has relevant keywords for discoverability', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd!.keywords).toContain('theme')
+      expect(cmd!.keywords).toContain('restore')
+      expect(cmd!.keywords).toContain('default')
+    })
+  })
 })
 
 describe('pluralizeType', () => {

src/hooks/useCommandRegistry.ts

@@ -60,6 +60,7 @@ interface CommandRegistryConfig {
   onOpenTheme?: (themeId: string) => void
   onRemoveActiveVault?: () => void
   onRestoreGettingStarted?: () => void
+  onRestoreDefaultThemes?: () => void
   isGettingStartedHidden?: boolean
   vaultCount?: number
 }
@@ -194,7 +195,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
     themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme,
     onCheckForUpdates,
     onCreateType,
-    onRemoveActiveVault, onRestoreGettingStarted, isGettingStartedHidden, vaultCount,
+    onRemoveActiveVault, onRestoreGettingStarted, onRestoreDefaultThemes, isGettingStartedHidden, vaultCount,
     mcpStatus, onInstallMcp,
   } = config
 
@@ -248,6 +249,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
 
       // Appearance
       ...buildThemeCommands(themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme),
+      { id: 'restore-default-themes', label: 'Restore Default Themes', group: 'Appearance', keywords: ['theme', 'reset', 'restore', 'default', 'fix', 'missing'], enabled: !!onRestoreDefaultThemes, execute: () => onRestoreDefaultThemes?.() },
 
       // Settings
       { id: 'open-settings', label: 'Open Settings', group: 'Settings', shortcut: '⌘,', keywords: ['preferences', 'config'], enabled: true, execute: onOpenSettings },
@@ -271,7 +273,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
     onZoomIn, onZoomOut, onZoomReset, zoomLevel,
     onSelect, onOpenDailyNote, onCloseTab,
     onGoBack, onGoForward, canGoBack, canGoForward,
-    vaultTypes, themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme,
+    vaultTypes, themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme, onRestoreDefaultThemes,
     onRemoveActiveVault, onRestoreGettingStarted, isGettingStartedHidden, vaultCount,
     mcpStatus, onInstallMcp,
   ])