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/l10n/app_en.arb

@@ -75,6 +75,8 @@
   "chatEdited": "edited",
   "errorTrustRootChanged": "The relay's trust root has changed! Refusing to communicate.",
   "errorRelayUnavailable": "Relay unavailable (sender certificate)",
+  "errorNotYetRegistered": "Šepot is still connecting to the relay — try sending again in a moment.",
+  "errorContactLinkIncomplete": "This link is incomplete — the other person created it before connecting. Ask them for a new sepot:// link.",
   "errorAttachmentTooLarge": "Attachment is too large (max {maxMb} MB)",
   "errorGroupNeedsMember": "A group needs at least 1 member",
   "errorGroupMaxMembers": "A group can have at most {maxMembers} members",
@@ -294,6 +296,8 @@
   "chatListArchive": "Archive",
   "qrShareTitle": "My Šepot ID",
   "qrIdentityNotFound": "Identity not found",
+  "qrShareConnectingTitle": "Connecting to the relay…",
+  "qrShareConnectingBody": "Your QR code and link will appear once Šepot connects for the first time. Just a moment — it only takes a few seconds.",
   "commonCopiedToClipboard": "Copied to clipboard",
   "qrShareInviteMessage": "Add me on Šepot!\n\n{shareString}",
   "commonShare": "Share",