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  can be changed or withdrawn; older clients see them as text.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/test/mentions_test.dart

@@ -0,0 +1,38 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/models/group.dart';
+import 'package:sepot/models/message.dart';
+
+/// Zmínky přicházejí od cizích klientů — vadný tvar nesmí rozbít zprávu
+/// a počet je shora omezený.
+void main() {
+  test('platné zmínky projdou', () {
+    expect(mentionsFromWire(['aa11', 'bb22']), ['aa11', 'bb22']);
+  });
+
+  test('ne-seznam a ne-řetězce se zahodí', () {
+    expect(mentionsFromWire('aa11'), isEmpty);
+    expect(mentionsFromWire(null), isEmpty);
+    expect(mentionsFromWire(['aa11', 42, null, {'x': 1}]), ['aa11']);
+  });
+
+  test('počet je omezený stropem členů skupiny', () {
+    final many = List.generate(Group.maxMembers + 50, (i) => 'fp$i');
+    expect(mentionsFromWire(many), hasLength(Group.maxMembers));
+  });
+
+  test('zmínky přežijí uložení a načtení zprávy', () {
+    final m = Message.text(
+      senderId: 'me',
+      recipientId: 'gid',
+      content: 'ahoj @Alice',
+      mentions: const ['alice-fp'],
+    );
+    expect(Message.fromJson(m.toJson()).mentions, ['alice-fp']);
+  });
+
+  test('zpráva bez zmínek je neukládá', () {
+    final m = Message.text(senderId: 'me', recipientId: 'x', content: 'a');
+    expect(m.toJson().containsKey('mentions'), isFalse);
+    expect(Message.fromJson(m.toJson()).mentions, isEmpty);
+  });
+}