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  can be changed or withdrawn; older clients see them as text.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

@@ -1,6 +1,7 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:screen_protector/screen_protector.dart';
 import 'package:sepot/core/backup/backup_service.dart';
+import 'package:sepot/core/services/messaging_service.dart';
 import 'package:sepot/providers/messaging_provider.dart';
 import 'package:sepot/providers/storage_provider.dart';
 
@@ -32,6 +33,29 @@ class ReadReceiptsNotifier extends AsyncNotifier<bool> {
 final readReceiptsProvider =
     AsyncNotifierProvider<ReadReceiptsNotifier, bool>(ReadReceiptsNotifier.new);
 
+/// „píše…“ — stejná pravidla jako potvrzení přečtení: výchozí vypnuto,
+/// reciproční (kdo nesdílí, nevidí).
+class TypingIndicatorsNotifier extends AsyncNotifier<bool> {
+  @override
+  Future<bool> build() async {
+    final storage = ref.read(storageProvider);
+    return (await storage.getSetting<bool>(
+            MessagingService.typingSettingKey)) ??
+        false;
+  }
+
+  Future<void> set(bool value) async {
+    await ref
+        .read(storageProvider)
+        .setSetting(MessagingService.typingSettingKey, value);
+    state = AsyncData(value);
+  }
+}
+
+final typingIndicatorsProvider =
+    AsyncNotifierProvider<TypingIndicatorsNotifier, bool>(
+        TypingIndicatorsNotifier.new);
+
 /// Zabezpečení obrazovky: blok screenshotů (Android FLAG_SECURE) + skrytí
 /// obsahu v přepínači appek (iOS). Default VYPNUTO — blokuje i vlastní
 /// screenshoty, tak ať se uživatel rozhodne sám.