fix: reliable first-message send, QR routing, auto-lock recovery

Fixes reported by testers and found in an app review:

- First message right after install no longer fails. Before the app
  finished registering with the relay, the outgoing message could crash on
  a null delivery address. The profile field is now optional and the send
  path throws a clear "still connecting, try again shortly" error while
  registration completes in the background.
- The QR code and share link now appear only once the delivery address is
  ready. Previously a link generated before the first connection lacked
  routing, so the other side could not send the first message. The screen
  now shows a connecting state and reactively swaps in the QR when ready.
- Incomplete contact links are rejected on import instead of failing at
  first send, with a clear prompt to ask for a fresh link.
- Auto-lock no longer silences the app: after locking and unlocking,
  messaging is restarted (it previously stayed dead until app restart).
- Group messages are marked failed on a non-per-member send error instead
  of hanging in the "sending" state forever.
- Hardened the relay client against a rare duplicate connection when
  toggling Tor or locking mid-connect, and fromShareString now rejects a
  delivery address of the wrong length.
- Pin unifiedpush below 6.2.0 to drop the webcrypto dependency (its native
  framework lacks LC_ENCRYPTION_INFO and blocked the iOS upload).
- Skip Android release lint (network dependency check hangs on the build
  host and adds no value for a Flutter app).

app/lib/core/errors.dart

@@ -26,6 +26,24 @@ class RelayUnavailableError extends SepotUserError {
   String toString() => 'relay unavailable';
 }
 
+/// Vlastní registrace na relay ještě nedoběhla — nemáme sender adresu.
+/// Přechodný stav po čerstvé instalaci; registrace běží na pozadí.
+class NotYetRegisteredError extends SepotUserError {
+  const NotYetRegisteredError();
+
+  @override
+  String toString() => 'not registered on relay yet';
+}
+
+/// Odkaz/QR kontaktu je neúplný (chybí routing aid+da) — vygenerovaný
+/// dřív, než měla protistrana dokončenou registraci na relay.
+class ContactLinkIncompleteError extends SepotUserError {
+  const ContactLinkIncompleteError();
+
+  @override
+  String toString() => 'contact link is missing routing (incomplete)';
+}
+
 /// Příloha přesahuje serverový limit.
 class AttachmentTooLargeError extends SepotUserError {
   const AttachmentTooLargeError(this.maxBytes);