feat(desktop): diagnostic + manual retry for legacy key migration

The 0.18.1 fix relied on an existing-device + present-stronghold-key match.
That fails for users who:
  - had multiple device registrations and only retain the latest device's
    private key in the local vault
  - had a vault wipe / fresh OS install at some point
  - have device rows that vanished server-side but keys still locally

Migration now scans conversation_keys for distinct un-migrated
recipient_device_ids visible to the user (RLS-filtered) and probes the
stronghold for each, regardless of whether the server still lists that
device. Result struct surfaces attempted/migrated/noKey/decryptFail/rpcFail
counters; SecurityCenter shows them via a new "Migration erneut ausführen"
button so users can self-diagnose without DevTools.

Also adds [crypto-migration] console.info breadcrumbs at every decision
point so a single F12 shows what happened.

apps/desktop/src/lib/userIdentity.ts

@@ -148,21 +148,84 @@ export async function resetIdentity(params: { userId: string; pin: string }): Pr
   return setup.recoveryCode ?? '';
 }
 
+// Returned by ensureLegacyMigrated and SecurityCenter's manual retry. Lets
+// the UI surface "X conv-keys re-wrapped, Y stuck because no key in vault."
+export interface LegacyMigrationReport {
+  serverDevices: number;
+  strongholdKeysFromServerDevices: number;
+  strongholdKeysFromBundleScan: number;
+  attempted: number;
+  migrated: number;
+  noStrongholdKey: number;
+  decryptFailed: number;
+  rpcFailed: number;
+}
+
 async function runLegacyMigration(
   userId: string,
   ownNewPriv: Uint8Array,
   ownNewPub: Uint8Array,
-): Promise<void> {
+): Promise<LegacyMigrationReport> {
+  const report: LegacyMigrationReport = {
+    serverDevices: 0,
+    strongholdKeysFromServerDevices: 0,
+    strongholdKeysFromBundleScan: 0,
+    attempted: 0,
+    migrated: 0,
+    noStrongholdKey: 0,
+    decryptFailed: 0,
+    rpcFailed: 0,
+  };
+
   const devices = await listOwnDevices(supabase);
-  if (devices.length === 0) return;
+  report.serverDevices = devices.length;
   const ownLegacyDevicePrivateKeys: Record<string, Uint8Array> = {};
+
+  // 1) Try every server-listed device first.
   for (const d of devices) {
     const k = await devLocalSecretStore.getSecret(`chatapp.priv.${userId}.${d.id}`);
     if (k) ownLegacyDevicePrivateKeys[d.id] = k;
   }
+  report.strongholdKeysFromServerDevices = Object.keys(ownLegacyDevicePrivateKeys).length;
+
+  // 2) Scan our visible un-migrated conversation_keys for distinct
+  // recipient_device_ids and probe stronghold for each. This catches the case
+  // where a device row was deleted server-side but its key remains locally,
+  // OR where listOwnDevices missed a device because of an RLS edge.
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const { data: scanRowsRaw } = await (supabase as any)
+    .from('conversation_keys')
+    .select('recipient_device_id')
+    .is('recipient_user_id', null)
+    .not('recipient_device_id', 'is', null);
+  const scanIds = Array.from(new Set(
+    ((scanRowsRaw ?? []) as { recipient_device_id: string }[])
+      .map((r) => r.recipient_device_id)
+      .filter((id): id is string => !!id),
+  ));
+  for (const id of scanIds) {
+    if (ownLegacyDevicePrivateKeys[id]) continue;
+    const k = await devLocalSecretStore.getSecret(`chatapp.priv.${userId}.${id}`);
+    if (k) {
+      ownLegacyDevicePrivateKeys[id] = k;
+      report.strongholdKeysFromBundleScan += 1;
+    }
+  }
+
+  console.info(
+    '[crypto-migration] vault scan:',
+    'serverDevices=' + report.serverDevices,
+    'keysFromServerList=' + report.strongholdKeysFromServerDevices,
+    'extraKeysFromBundleScan=' + report.strongholdKeysFromBundleScan,
+  );
+
   const ids = Object.keys(ownLegacyDevicePrivateKeys);
-  if (ids.length === 0) return;
-  await migrateOwnLegacyBundles({
+  if (ids.length === 0) {
+    console.warn('[crypto-migration] no legacy private keys in vault — nothing to migrate');
+    return report;
+  }
+
+  const result = await migrateOwnLegacyBundles({
     client: supabase,
     ownUserId: userId,
     ownNewPublicKey: ownNewPub,
@@ -170,6 +233,22 @@ async function runLegacyMigration(
     ownLegacyDeviceIds: ids,
     ownLegacyDevicePrivateKeys,
   });
+
+  report.attempted = result.attempted;
+  report.migrated = result.migratedConversations;
+  report.noStrongholdKey = result.noStrongholdKey;
+  report.decryptFailed = result.decryptFailed;
+  report.rpcFailed = result.rpcFailed;
+  return report;
+}
+
+// Public wrapper for SecurityCenter's "Migration erneut versuchen" button.
+// Returns a structured report so the UI can render numbers and reasons.
+export async function retryLegacyMigration(userId: string): Promise<LegacyMigrationReport> {
+  const priv = await devLocalSecretStore.getSecret(cacheKey(userId));
+  if (!priv) throw new Error('user key not cached locally — re-login required');
+  const pub = await derivePublicKey(priv);
+  return runLegacyMigration(userId, priv, pub);
 }
 
 async function derivePublicKey(privateKey: Uint8Array): Promise<Uint8Array> {