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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,79 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:sepot/core/crypto/attachment_cipher.dart';
+import 'package:sepot/core/storage/attachment_store.dart';
+
+/// Odkaz na profilovou fotku: šifrovaný blob na relayi + klíč k němu.
+///
+/// Fotka sama nejde v každé zprávě — v profilu `p` cestuje jen tenhle
+/// odkaz (pár set bajtů). Příjemce blob stáhne jednou, ověří digest
+/// a drží si ho šifrovaný u sebe.
+class AvatarRef {
+  const AvatarRef({
+    required this.blobId,
+    required this.key,
+    required this.nonce,
+    required this.digest,
+    this.uploadedAt,
+  });
+
+  /// Profilová fotka je malá (256 px JPEG) — cokoli většího je podvrh.
+  static const maxBlobBytes = 256 * 1024;
+
+  final String blobId;
+  final Uint8List key;
+  final Uint8List nonce;
+  final Uint8List digest;
+
+  /// Kdy jsem blob nahrál (jen u vlastní fotky — kvůli obnově před
+  /// vypršením na relayi).
+  final DateTime? uploadedAt;
+
+  Map<String, dynamic> toWire() => {
+        'b': blobId,
+        'k': base64Encode(key),
+        'n': base64Encode(nonce),
+        'd': base64Encode(digest),
+      };
+
+  /// Odkaz od protistrany; cokoli podezřelého = null (fotka se neukáže).
+  static AvatarRef? fromWire(Object? raw) {
+    if (raw is! Map) return null;
+    final b = raw['b'];
+    if (b is! String || !AttachmentStore.isValidBlobId(b)) return null;
+    try {
+      final key = base64Decode(raw['k'] as String);
+      final nonce = base64Decode(raw['n'] as String);
+      final digest = base64Decode(raw['d'] as String);
+      if (key.length != AttachmentCipher.keyLength ||
+          nonce.length != AttachmentCipher.nonceLength ||
+          digest.length != 32) {
+        return null;
+      }
+      return AvatarRef(blobId: b, key: key, nonce: nonce, digest: digest);
+    } on Object {
+      return null;
+    }
+  }
+
+  Map<String, dynamic> toJson() => {
+        ...toWire(),
+        if (uploadedAt != null) 'u': uploadedAt!.millisecondsSinceEpoch,
+      };
+
+  static AvatarRef? fromJson(Object? raw) {
+    final ref = fromWire(raw);
+    if (ref == null || raw is! Map) return ref;
+    final u = raw['u'];
+    return u is int
+        ? AvatarRef(
+            blobId: ref.blobId,
+            key: ref.key,
+            nonce: ref.nonce,
+            digest: ref.digest,
+            uploadedAt: DateTime.fromMillisecondsSinceEpoch(u),
+          )
+        : ref;
+  }
+}