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/auth/userKey.ts

@@ -92,7 +92,13 @@ export async function uploadUserKeyBlob(
   client: AppSupabaseClient,
   params: UploadParams,
 ): Promise<void> {
-  const { error } = await rpc(client).rpc('reset_user_key', {
+  // Non-destructive UPSERT — must NOT touch conversation_keys. Used for the
+  // first-time PIN setup, PIN change, and recovery-code regeneration. The
+  // 0.18.0–0.18.2 builds wired this to `reset_user_key` which DELETED every
+  // legacy conv-key bundle for the user before the migration could re-wrap
+  // them, leaving people unable to read or send. `upsert_user_key` writes
+  // only the user_keys row and leaves conversation_keys alone.
+  const { error } = await rpc(client).rpc('upsert_user_key', {
     p_user_id: params.userId,
     p_public_key_b64: bytesToB64(params.publicKey),
     p_sealed_private_b64: bytesToB64(params.sealedPrivateKey),