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  can be changed or withdrawn; older clients see them as text.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

@@ -1,5 +1,8 @@
+import 'dart:typed_data';
+
 import 'package:flutter/widgets.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:sepot/core/calls/call_controller.dart';
 import 'package:sepot/core/network/relay_client.dart';
 import 'package:sepot/core/push/push_notifier.dart';
 import 'package:sepot/core/services/messaging_service.dart';
@@ -16,6 +19,28 @@ final messagingServiceProvider = Provider<MessagingService>((ref) {
   return service;
 });
 
+/// Kdo právě píše: konverzace → otisky pisatelů. Prázdné, když je
+/// „píše…“ vypnuté (služba pak nic nepřijímá).
+final typingProvider = StreamProvider<Map<String, Set<String>>>((ref) async* {
+  final tracker = ref.watch(messagingServiceProvider).typing;
+  yield tracker.current;
+  yield* tracker.changes;
+});
+
+/// Profilová fotka člověka podle otisku (null = iniciály). Přepočítá se,
+/// když se změní kontakty — tam dorazí nový odkaz i stažení fotky.
+final avatarProvider =
+    FutureProvider.family<Uint8List?, String>((ref, fingerprint) async {
+  final service = ref.watch(messagingServiceProvider);
+  final me = ref.watch(identityProvider).valueOrNull;
+  final contacts = ref.watch(contactsProvider).valueOrNull ?? const [];
+  if (!service.isStarted) return null;
+  final avatar = fingerprint == me?.id
+      ? service.myAvatar
+      : contacts.where((c) => c.id == fingerprint).firstOrNull?.avatar;
+  return avatar == null ? null : service.loadAvatar(avatar);
+});
+
 /// Nastartuje službu, jakmile existuje identita. Watchovat z první
 /// obrazovky po odemčení (chat list) — do té doby se nic nespojuje.
 /// Předpokládá odemčený storage — na /chats se jde vždy až po unlocku.
@@ -46,16 +71,21 @@ final messagingStartProvider = FutureProvider<void>((ref) async {
 
   // Appka běží, ale není v popředí (jiná appka / zhasnutý displej se
   // živým procesem) → systémová notifikace. Jen jméno, žádný obsah.
-  // Ztlumená konverzace notifikaci potlačí (unread badge běží dál).
-  service.onMessageReceived = (contact, message) async {
+  // Ztlumená konverzace notifikaci potlačí (unread badge běží dál),
+  // zmínka mě ve skupině projde i tak.
+  service.onMessageReceived = (contact, message, convId) async {
     final visible = WidgetsBinding.instance.lifecycleState ==
         AppLifecycleState.resumed;
     if (visible) return;
-    final conv = await ref.read(storageProvider).loadConversation(contact.id);
-    if (conv?.isMuted == true) return;
+    final conv = await ref.read(storageProvider).loadConversation(convId);
+    final mentionedMe = message.mentions.contains(identity.id);
+    if (conv?.isMuted == true && !mentionedMe) return;
     PushNotifier.showMessageNotification(contact.displayName);
   };
 
+  service.onCallSignal = (contact, signal) =>
+      ref.read(callProvider.notifier).onSignal(contact, signal);
+
   await service.start(identity);
 });