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  can be changed or withdrawn; older clients see them as text.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/models/contact.dart

@@ -1,3 +1,4 @@
+import 'package:sepot/models/avatar_ref.dart';
 import 'package:sepot/models/identity.dart';
 
 /// Kontakt — osoba, se kterou můžeme chatovat
@@ -14,6 +15,9 @@ class Contact {
   /// nezobrazí) a odesílání je v UI zablokované.
   bool blocked;
 
+  /// Profilová fotka, kterou kontakt sdílí (null = iniciály).
+  AvatarRef? avatar;
+
   Contact({
     required this.displayName,
     required this.publicIdentity,
@@ -21,6 +25,7 @@ class Contact {
     this.verified = false,
     this.notes,
     this.blocked = false,
+    this.avatar,
   });
 
   /// ID kontaktu = fingerprint identity klíče (identita = klíč).
@@ -58,6 +63,7 @@ class Contact {
         'verified': verified,
         'notes': notes,
         'blocked': blocked,
+        'avatar': avatar?.toJson(),
       };
 
   factory Contact.fromJson(Map<String, dynamic> json) {
@@ -70,6 +76,7 @@ class Contact {
       verified: json['verified'] as bool? ?? false,
       notes: json['notes'] as String?,
       blocked: json['blocked'] as bool? ?? false,
+      avatar: AvatarRef.fromJson(json['avatar']),
     );
   }
 }