feat(app): redesigned interface, polls, drafts, forwarding and mentions

A new look across the app and a set of everyday messaging features.

Interface
- Home screen with Chats, Contacts and Settings tabs, a large title,
  search across chats and message text (diacritics-insensitive), filters
  and pinned chats.
- Conversation view with a translucent header and composer, grouped
  bubbles, author avatars and colours in groups, swipe to reply, an
  "N new messages" marker, a jump-to-latest button and in-chat search.
- Contact and group profiles: verification, shared photos, groups in
  common, disappearing messages, mute, block; members, admin badges,
  adding people, rename and leave for groups.
- Full-screen photo viewer that pages through the whole conversation.
- A stable avatar colour per person and a clearly visible read state.

Messaging
- Polls in chats and groups, single or multiple choice, with votes that
  can be changed or withdrawn; older clients see them as text.
- Drafts survive leaving a chat and show in the chat list.
- Forwarding of messages, photos, files and voice messages.
- Notes to self: a local-only, encrypted chat that never reaches the
  relay.
- @mentions in groups that notify even when the group is muted.
- Typing indicators sent as ephemeral messages, opt-in and mutual like
  read receipts.
- Delivery and read times per message and per group member.
- Profile photos, cropped, resized and stripped of metadata, delivered
  encrypted to contacts.
- Voice message playback at 1.5x and 2x.
- Call buttons in 1:1 chats and call log entries.

Fixes
- Muted groups no longer notify: muting was checked against the
  author's chat instead of the group.
- A blocked contact could unblock itself by changing its delivery
  address.
- Read receipts no longer regress to delivered when a late delivery
  receipt arrives.
- A message marked as failed corrects itself once it is delivered.
- Double ticks in groups now mean delivered to every member.

app/lib/core/network/relay_client.dart

@@ -63,6 +63,7 @@ class RelayClient {
       StreamController<RelayConnectionState>.broadcast();
   final _deliverController = StreamController<pb.Envelope>.broadcast();
   final _receiptController = StreamController<pb.Receipt>.broadcast();
+  final _turnController = StreamController<pb.TurnCredentials>.broadcast();
 
   var _state = RelayConnectionState.disconnected;
 
@@ -340,6 +341,10 @@ class RelayClient {
             if (!_receiptController.isClosed) {
               _receiptController.add(frame.receipt);
             }
+          } else if (frame.hasTurnCredentials()) {
+            if (!_turnController.isClosed) {
+              _turnController.add(frame.turnCredentials);
+            }
           }
         },
         onDone: _onDisconnected,
@@ -373,10 +378,11 @@ class RelayClient {
   }
 
   /// Odešle sealed-sender obálku. Vrací Receipt (STORED_OFFLINE /
-  /// DELIVERED_ONLINE) spárovaný přes msg_uuid.
+  /// DELIVERED_ONLINE, u EPHEMERAL i DROPPED) spárovaný přes msg_uuid.
   Future<pb.Receipt> sendEnvelope({
     required Uint8List dstAddress,
     required Uint8List payload,
+    pb.ContentHint hint = pb.ContentHint.CONTENT_HINT_DEFAULT,
     Duration timeout = const Duration(seconds: 10),
   }) async {
     final msgUuid = randomBytes(16);
@@ -387,13 +393,24 @@ class RelayClient {
       envelope: pb.Envelope(
         dstAddress: dstAddress,
         msgUuid: msgUuid,
-        contentHint: pb.ContentHint.CONTENT_HINT_DEFAULT,
+        contentHint: hint,
         payload: payload,
       ),
     ));
     return receiptFuture;
   }
 
+  /// Dočasné údaje k TURN serveru pro hovor. Relay je vydá jen přihlášenému
+  /// zařízení; bez TURN na relayi (503) doběhne timeout a hovor zkusí jen
+  /// přímé spojení přes STUN.
+  Future<pb.TurnCredentials> requestTurn({
+    Duration timeout = const Duration(seconds: 5),
+  }) {
+    final future = _turnController.stream.first.timeout(timeout);
+    _send(pb.ClientFrame(turnRequest: pb.TurnRequest()));
+    return future;
+  }
+
   /// Potvrdí zpracování obálky — relay ji smaže z fronty. Volat až PO
   /// uložení zprávy na disk.
   void ack(List<int> msgUuid) {
@@ -437,6 +454,7 @@ class RelayClient {
     await stop();
     _transport.close();
     await _stateController.close();
+    await _turnController.close();
     await _deliverController.close();
     await _receiptController.close();
   }