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  can be changed or withdrawn; older clients see them as text.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

@@ -1,4 +1,7 @@
 import 'package:sepot/models/attachment.dart';
+import 'package:sepot/models/call_log.dart';
+import 'package:sepot/models/group.dart';
+import 'package:sepot/models/poll.dart';
 import 'package:uuid/uuid.dart';
 
 /// Typy zpráv — odpovídá Rust: MessageType
@@ -11,8 +14,8 @@ enum MessageType {
   keyExchange,
 }
 
-/// Stav odeslané (mojí) zprávy — poctivá hranice: read receipts zatím
-/// NEjsou (design: default OFF), takže „přečteno" se nezobrazuje.
+/// Stav odeslané (mojí) zprávy. Potvrzení přečtení je volitelné (default
+/// vypnuto) a reciproční — kdo ho nesdílí, „přečteno" neuvidí ani sám.
 ///   sending   — lokálně zařazeno, čeká na potvrzení relaye (⏱)
 ///   sent      — relay přijal, příjemce byl offline → v jeho frontě (✓)
 ///   delivered — doručeno na zařízení příjemce (✓✓)
@@ -43,6 +46,21 @@ class Message {
   final bool isDeleted; // smazáno pro všechny (tombstone)
   /// Reakce: fingerprint odesílatele reakce → emoji. Prázdné = žádné.
   final Map<String, String> reactions;
+  /// Kdy mou zprávu dostal / přečetl který příjemce (fingerprint → čas).
+  /// 1:1 má nejvýš jeden záznam, skupina jeden na člena — z toho se
+  /// skládá „Info o zprávě". Čas je z hodin příjemce (pole `ts` doručenky).
+  final Map<String, DateTime> deliveredTo;
+  final Map<String, DateTime> readBy;
+
+  /// Anketa (textová zpráva, `content` nese záložní text pro starší appky).
+  final Poll? poll;
+
+  /// Fingerprinty lidí zmíněných přes @ ve skupině — zmínka projde i přes
+  /// ztlumení skupiny.
+  final List<String> mentions;
+
+  /// Lokální záznam hovoru — taková zpráva nemá text ani přílohu.
+  final CallLog? callLog;
 
   const Message({
     required this.id,
@@ -62,6 +80,11 @@ class Message {
     this.editedAt,
     this.isDeleted = false,
     this.reactions = const {},
+    this.deliveredTo = const {},
+    this.readBy = const {},
+    this.poll,
+    this.mentions = const [],
+    this.callLog,
   });
 
   /// Vytvoř textovou zprávu
@@ -72,6 +95,8 @@ class Message {
     String? replyToId,
     int? disappearAfterSeconds,
     MessageStatus status = MessageStatus.sending,
+    Poll? poll,
+    List<String> mentions = const [],
   }) {
     return Message(
       id: const Uuid().v4(),
@@ -83,6 +108,8 @@ class Message {
       timestamp: DateTime.now(),
       status: status,
       disappearAfterSeconds: disappearAfterSeconds,
+      poll: poll,
+      mentions: mentions,
     );
   }
 
@@ -137,6 +164,9 @@ class Message {
     DateTime? editedAt,
     bool? isDeleted,
     Map<String, String>? reactions,
+    Map<String, DateTime>? deliveredTo,
+    Map<String, DateTime>? readBy,
+    Poll? poll,
   }) {
     return Message(
       id: id,
@@ -156,9 +186,22 @@ class Message {
       editedAt: editedAt ?? this.editedAt,
       isDeleted: isDeleted ?? this.isDeleted,
       reactions: reactions ?? this.reactions,
+      deliveredTo: deliveredTo ?? this.deliveredTo,
+      readBy: readBy ?? this.readBy,
+      poll: poll ?? this.poll,
+      mentions: mentions,
+      callLog: callLog,
     );
   }
 
+  /// Nejdřívější doručení / přečtení napříč příjemci (null = zatím nic).
+  DateTime? get firstDeliveredAt => _earliest(deliveredTo.values);
+  DateTime? get firstReadAt => _earliest(readBy.values);
+
+  static DateTime? _earliest(Iterable<DateTime> times) => times.isEmpty
+      ? null
+      : times.reduce((a, b) => a.isBefore(b) ? a : b);
+
   /// Má zpráva mizet?
   bool get isDisappearing => disappearAfterSeconds != null;
 
@@ -183,8 +226,27 @@ class Message {
         'editedAt': editedAt?.toIso8601String(),
         'isDeleted': isDeleted,
         'reactions': reactions,
+        'deliveredTo': _timesToJson(deliveredTo),
+        'readBy': _timesToJson(readBy),
+        'poll': poll?.toJson(),
+        if (mentions.isNotEmpty) 'mentions': mentions,
+        if (callLog != null) 'call': callLog!.toJson(),
       };
 
+  static Map<String, int> _timesToJson(Map<String, DateTime> times) =>
+      times.map((k, v) => MapEntry(k, v.millisecondsSinceEpoch));
+
+  /// Tolerantní čtení: starší záznamy pole nemají, vadné položky se
+  /// přeskočí — kvůli nim nesmí spadnout načtení celé konverzace.
+  static Map<String, DateTime> _timesFromJson(Object? raw) {
+    if (raw is! Map) return const {};
+    return {
+      for (final e in raw.entries)
+        if (e.key is String && e.value is int)
+          e.key as String: DateTime.fromMillisecondsSinceEpoch(e.value as int),
+    };
+  }
+
   factory Message.fromJson(Map<String, dynamic> json) {
     return Message(
       id: json['id'] as String,
@@ -215,6 +277,18 @@ class Message {
       reactions: (json['reactions'] as Map<String, dynamic>?)
               ?.map((k, v) => MapEntry(k, v as String)) ??
           const {},
+      deliveredTo: _timesFromJson(json['deliveredTo']),
+      readBy: _timesFromJson(json['readBy']),
+      poll: Poll.fromJson(json['poll']),
+      mentions: mentionsFromWire(json['mentions']),
+      callLog: CallLog.fromJson(json['call']),
     );
   }
 }
+
+/// Zmínky z drátu i z úložiště: jen řetězce a rozumný počet — obálka od
+/// cizího klienta nesmí rozbít zprávu ani nafouknout úložiště.
+List<String> mentionsFromWire(Object? raw) {
+  if (raw is! List) return const [];
+  return raw.whereType<String>().take(Group.maxMembers).toList();
+}