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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,65 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/core/services/typing_tracker.dart';
+
+/// „píše…“ nemá zprávu „dopsal jsem“ — indikátor musí sám vyhasnout,
+/// jinak by po ztraceném spojení svítil navždy.
+void main() {
+  late DateTime now;
+  late TypingTracker tracker;
+
+  setUp(() {
+    now = DateTime(2026, 9, 29, 12);
+    tracker = TypingTracker(clock: () => now);
+  });
+
+  tearDown(() => tracker.dispose());
+
+  test('pisatel se ukáže v dané konverzaci', () {
+    tracker.mark('chat-a', 'alice');
+    expect(tracker.current, {
+      'chat-a': {'alice'},
+    });
+  });
+
+  test('bez dalšího „píšu“ indikátor vyprší', () {
+    tracker.mark('chat-a', 'alice');
+    now = now.add(TypingTracker.showFor + const Duration(milliseconds: 1));
+    expect(tracker.current, isEmpty);
+    tracker.prune();
+    expect(tracker.current, isEmpty);
+  });
+
+  test('opakované „píšu“ prodlužuje', () {
+    tracker.mark('chat-a', 'alice');
+    now = now.add(const Duration(seconds: 4));
+    tracker.mark('chat-a', 'alice');
+    now = now.add(const Duration(seconds: 4));
+    expect(tracker.current['chat-a'], {'alice'});
+  });
+
+  test('příchozí zpráva indikátor hned zhasne', () {
+    tracker
+      ..mark('skupina', 'alice')
+      ..mark('skupina', 'tomas')
+      ..clear('skupina', 'alice');
+    expect(tracker.current, {
+      'skupina': {'tomas'},
+    });
+  });
+
+  test('změny se hlásí posluchačům', () async {
+    final events = <Map<String, Set<String>>>[];
+    final sub = tracker.changes.listen(events.add);
+    tracker
+      ..mark('chat-a', 'alice')
+      ..clear('chat-a', 'alice');
+    await Future<void>.delayed(Duration.zero);
+    await sub.cancel();
+    expect(events, [
+      {
+        'chat-a': {'alice'},
+      },
+      <String, Set<String>>{},
+    ]);
+  });
+}