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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,71 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/models/message.dart';
+import 'package:sepot/models/poll.dart';
+
+/// Anketa přichází od protistrany — vadná data nesmí nic rozbít a hlas
+/// nesmí jít mimo nabídnuté možnosti.
+void main() {
+  const poll = Poll(
+    question: 'Kdy vyrazíme?',
+    options: ['Pátek', 'Sobota', 'Neděle'],
+  );
+
+  test('drát tam a zpět zachová otázku i možnosti', () {
+    final back = Poll.fromWire(poll.toWire())!;
+    expect(back.question, poll.question);
+    expect(back.options, poll.options);
+    expect(back.multi, isFalse);
+  });
+
+  test('vadná anketa z drátu = null (ukáže se záložní text)', () {
+    expect(Poll.fromWire(null), isNull);
+    expect(Poll.fromWire({'q': 'Jen jedna?', 'o': ['a']}), isNull);
+    expect(Poll.fromWire({'q': '', 'o': ['a', 'b']}), isNull);
+    expect(Poll.fromWire({'q': 'x', 'o': ['a', 42]}), isNull);
+    expect(
+        Poll.fromWire({'q': 'x', 'o': List.filled(Poll.maxOptions + 1, 'a')}),
+        isNull);
+    expect(
+        Poll.fromWire({'q': 'x' * (Poll.maxQuestionLength + 1), 'o': ['a', 'b']}),
+        isNull);
+  });
+
+  test('hlas mimo nabídku se zahodí', () {
+    final p = poll.withVote('bob', [7, -1]);
+    expect(p.votes, isEmpty);
+  });
+
+  test('jedna volba: platí jen první index', () {
+    final p = poll.withVote('bob', [2, 0]);
+    expect(p.votes['bob'], [0]);
+  });
+
+  test('více voleb: duplicity se sloučí', () {
+    const multi = Poll(question: 'Co vezmeme?', options: ['a', 'b', 'c'], multi: true);
+    final p = multi.withVote('bob', [2, 0, 2]);
+    expect(p.votes['bob'], [0, 2]);
+    expect(p.countFor(2), 1);
+  });
+
+  test('nový hlas přepíše starý, prázdný ho zruší', () {
+    var p = poll.withVote('bob', [0]).withVote('cyril', [0]);
+    expect(p.countFor(0), 2);
+    p = p.withVote('bob', [1]);
+    expect(p.countFor(0), 1);
+    expect(p.countFor(1), 1);
+    p = p.withVote('bob', const []);
+    expect(p.voterCount, 1);
+  });
+
+  test('hlasy přežijí uložení zprávy', () {
+    final msg = Message.text(
+      senderId: 'ja',
+      recipientId: 'chata',
+      content: 'Anketa: Kdy vyrazíme?',
+      poll: poll.withVote('bob', [1]),
+    );
+    final back = Message.fromJson(msg.toJson());
+    expect(back.poll?.question, poll.question);
+    expect(back.poll?.votes['bob'], [1]);
+  });
+}