fix: persist vault list across app updates

Vault list was stored only in React useState, lost on every app restart
or update. Now persisted to ~/.config/com.laputa.app/vaults.json via
Rust backend commands (load_vault_list, save_vault_list).

- Add vault_list.rs module with VaultEntry/VaultList types and JSON I/O
- Register load_vault_list/save_vault_list Tauri commands
- Extract vaultListStore.ts utility for frontend persistence calls
- Rewrite useVaultSwitcher to load on mount and persist on change
- Show unavailable vaults greyed out with warning icon instead of
  silently removing them
- Add mock handlers for browser/test environments
- Add useVaultSwitcher tests covering persistence, availability, dedup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/vaultListStore.ts

@@ -0,0 +1,36 @@
+import { invoke } from '@tauri-apps/api/core'
+import { isTauri, mockInvoke } from '../mock-tauri'
+import type { VaultOption } from '../components/StatusBar'
+
+export interface PersistedVaultList {
+  vaults: Array<{ label: string; path: string }>
+  active_vault: string | null
+}
+
+function tauriCall<T>(command: string, args: Record<string, unknown>): Promise<T> {
+  return isTauri() ? invoke<T>(command, args) : mockInvoke<T>(command, args)
+}
+
+async function checkAvailability(v: { label: string; path: string }): Promise<VaultOption> {
+  try {
+    const exists = await tauriCall<boolean>('check_vault_exists', { path: v.path })
+    return { label: v.label, path: v.path, available: exists }
+  } catch {
+    return { label: v.label, path: v.path, available: false }
+  }
+}
+
+export async function loadVaultList(): Promise<{ vaults: VaultOption[]; activeVault: string | null }> {
+  const data = await tauriCall<PersistedVaultList>('load_vault_list', {})
+  const persisted = data?.vaults ?? []
+  const checked = await Promise.all(persisted.map(checkAvailability))
+  return { vaults: checked, activeVault: data?.active_vault ?? null }
+}
+
+export function saveVaultList(vaults: VaultOption[], activeVault: string): Promise<void> {
+  const list: PersistedVaultList = {
+    vaults: vaults.map(v => ({ label: v.label, path: v.path })),
+    active_vault: activeVault,
+  }
+  return tauriCall('save_vault_list', { list })
+}