feat(crypto): stronghold persistence + passphrase device-key backup/restore

apps/desktop/src/lib/secretStore.ts

@@ -1,12 +1,21 @@
 import { base64FromBytes, bytesFromBase64, type SecretStore } from '@chat-app/shared/auth';
 
-// M1 dev-only impl: persists secrets as base64 in localStorage.
-// Swap this out for a tauri-plugin-stronghold implementation before release.
-// The SecretStore interface stays identical so callers won't notice.
+import { isTauriRuntime } from './globalShortcut';
+import { makeStrongholdStore, migrateLocalStorageToStronghold } from './strongholdStore';
+
+// Secret store with two backends:
+//   - Tauri: Stronghold-encrypted vault file in appLocalDataDir. Survives app
+//     reinstalls and is encrypted at rest with a password derived from the
+//     authenticated user-id.
+//   - Web / pre-auth: localStorage (legacy dev fallback).
+//
+// Callers don't need to care which one is active — they import a singleton
+// and call setSecretStoreUser(userId) once the session is known. Until that
+// happens, calls fall through to localStorage.
 
 const PREFIX = 'chatapp.secret:';
 
-export const devLocalSecretStore: SecretStore = {
+const localStore: SecretStore = {
   async getSecret(key: string): Promise<Uint8Array | null> {
     const raw = window.localStorage.getItem(PREFIX + key);
     if (!raw) return null;
@@ -20,3 +29,42 @@ export const devLocalSecretStore: SecretStore = {
     window.localStorage.removeItem(PREFIX + key);
   },
 };
+
+let activeBackend: SecretStore = localStore;
+let activeUserId: string | null = null;
+
+export async function setSecretStoreUser(userId: string | null): Promise<void> {
+  if (userId === activeUserId) return;
+  activeUserId = userId;
+
+  if (userId && isTauriRuntime()) {
+    const stronghold = makeStrongholdStore(userId);
+    activeBackend = stronghold;
+    try {
+      await migrateLocalStorageToStronghold(userId, PREFIX);
+    } catch (err: unknown) {
+      console.warn('stronghold migration failed', err);
+    }
+  } else {
+    activeBackend = localStore;
+  }
+}
+
+// Singleton with stable identity — internals delegate to whichever backend is
+// currently active. Existing call-sites that imported `devLocalSecretStore`
+// keep working without changes.
+export const devLocalSecretStore: SecretStore = {
+  async getSecret(key) {
+    return activeBackend.getSecret(key);
+  },
+  async setSecret(key, value) {
+    return activeBackend.setSecret(key, value);
+  },
+  async removeSecret(key) {
+    return activeBackend.removeSecret(key);
+  },
+};
+
+export function isStrongholdActive(): boolean {
+  return activeBackend !== localStore;
+}