fix: stop reset_user_key from wiping conv-key bundles + auto-rotate stuck convs

Root cause of "alle Nachrichten verschlüsselt + kann nicht schreiben":
uploadUserKeyBlob (called by setupNewUserIdentity, changePin and
regenerateRecoveryCode) routed through reset_user_key, which DELETES
every conversation_keys row addressed to the user or one of their
devices. So setting a PIN destroyed every legacy bundle BEFORE the
migration could re-wrap them. The user ended up with user_keys set,
zero un-migrated bundles, no decryption, no send.

Fixes shipped:

  * supabase/migrations/20260516000001_user_key_rpcs_v2.sql
    - upsert_user_key: same UPSERT, NO delete. Used everywhere except
      "Identität zurücksetzen" (which keeps reset_user_key on purpose).
    - rotate_conv_key: bumps active_key_version atomically and inserts
      a fresh batch of bundles (per-user + per-device fallback).
  * shared/auth/userKey.ts: uploadUserKeyBlob now calls upsert_user_key.
  * shared/chat/convKeys.ts: new rotateConvKey() that wraps the fresh
    conv-key for every member's user_keys (preferred) and falls back to
    each member's per-device public_key for peers still on 0.17.x.
  * shared/chat/convKeys.ts: getOrCreateConvKey auto-triggers rotate
    when the user has no recipient_user_id row at the active version
    but rows exist (the deadlock case). Existing outbox retries drain
    on their own once the rotate completes — no manual button.
  * desktop/MessageBubble.tsx: "...cannot decrypt" is now a softer,
    German "Nachricht nicht lesbar" so users don't think the app
    crashed when historical messages can't be unwrapped.

packages/shared/src/chat/convKeys.ts

@@ -138,11 +138,105 @@ export async function getOrCreateConvKey(
     .eq('key_version', version);
   if (cntErr) throw cntErr;
   if ((count ?? 0) > 0) {
-    throw new Error('Awaiting conversation key — another user must share it with this user.');
+    // Rows exist for this version, but none for me. Either I lost the device-key
+    // that originally received my bundle, or my own bundle was wiped by the
+    // 0.18.0 reset_user_key bug. Either way, the only way out is to mint a fresh
+    // conv-key at version+1 and wrap it for everyone we can. Old messages stay
+    // unreadable for me; new ones flow.
+    console.info('[conv-key] no bundle for me at v' + version + ' — auto-rotating');
+    return rotateConvKey(client, conversationId, own);
   }
   return bootstrapConvKey(client, conversationId, own, version);
 }
 
+// Mints a fresh conv-key at active_key_version + 1 and wraps it for every
+// accepted member. Per-user bundles take priority; for members lacking a
+// user_keys row we fall back to per-device wrapping (one bundle per device)
+// so peers still on the legacy 0.17.x client can decrypt with their device
+// private key. Caller must own a copy of their private key in `own`.
+export async function rotateConvKey(
+  client: AppSupabaseClient,
+  conversationId: string,
+  own: OwnUserCtx,
+): Promise<ConvKeyHandle> {
+  const currentVersion = await fetchActiveKeyVersion(client, conversationId);
+  const newVersion = currentVersion + 1;
+
+  const { data: members, error: mErr } = await client
+    .from('conversation_members')
+    .select('user_id, accepted')
+    .eq('conversation_id', conversationId);
+  if (mErr) throw mErr;
+  const memberIds = (members ?? []).filter((m) => m.accepted).map((m) => m.user_id);
+  if (memberIds.length === 0) throw new Error('cannot rotate — no accepted members');
+
+  const userKeys = await fetchPeerPublicKeys(client, memberIds);
+  const userKeyByUserId = new Map(userKeys.map((k) => [k.userId, k.publicKey]));
+  const missingUserKeyMembers = memberIds.filter((id) => !userKeyByUserId.has(id));
+
+  let legacyDevices: { userId: string; deviceId: string; publicKey: Uint8Array }[] = [];
+  if (missingUserKeyMembers.length > 0) {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const { data: devs, error: dErr } = await (client as any)
+      .from('devices')
+      .select('id, user_id, public_key')
+      .in('user_id', missingUserKeyMembers)
+      .not('public_key', 'is', null);
+    if (dErr) throw dErr;
+    legacyDevices = (devs ?? []).map((d: { id: string; user_id: string; public_key: string }) => ({
+      userId: d.user_id,
+      deviceId: d.id,
+      publicKey: pgHexToBytes(d.public_key),
+    }));
+  }
+
+  const convKey = generateConvKey();
+  const bundles: Array<{
+    recipient_user_id?: string;
+    recipient_device_id?: string;
+    encrypted_key: string;
+    nonce: string;
+  }> = [];
+  for (const k of userKeys) {
+    const wrapped = await wrapConvKeyForRecipient(convKey, k.publicKey, own.privateKey);
+    bundles.push({
+      recipient_user_id: k.userId,
+      encrypted_key: hexNoPrefix(wrapped.ciphertext),
+      nonce: hexNoPrefix(wrapped.nonce),
+    });
+  }
+  for (const d of legacyDevices) {
+    const wrapped = await wrapConvKeyForRecipient(convKey, d.publicKey, own.privateKey);
+    bundles.push({
+      recipient_device_id: d.deviceId,
+      encrypted_key: hexNoPrefix(wrapped.ciphertext),
+      nonce: hexNoPrefix(wrapped.nonce),
+    });
+  }
+  if (bundles.length === 0) {
+    throw new Error('cannot rotate — no peers have a public key (no user_keys, no devices)');
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const rpc = (client as unknown as { rpc: (n: string, p: object) => Promise<{ error: any }> }).rpc;
+  const { error } = await rpc.call(client, 'rotate_conv_key', {
+    p_conv_id: conversationId,
+    p_sender_user_id: own.userId,
+    p_new_version: newVersion,
+    p_bundles: bundles,
+  });
+  if (error) throw error;
+
+  const handle = { conversationId, keyVersion: newVersion, key: convKey };
+  cache.set(cacheKey(conversationId, newVersion), handle);
+  console.info(
+    '[conv-key] rotated conversation ' + conversationId.slice(0, 8) +
+    ' from v' + currentVersion + ' to v' + newVersion +
+    ' — wrapped for ' + userKeys.length + ' user-keys + ' + legacyDevices.length + ' legacy devices',
+  );
+  return handle;
+}
+
 export async function tryGetConvKey(
   client: AppSupabaseClient,
   conversationId: string,