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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,121 @@
+import 'dart:typed_data';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sepot/features/profile/group_profile_screen.dart';
+import 'package:sepot/features/profile/profile_widgets.dart';
+import 'package:sepot/l10n/generated/app_localizations.dart';
+import 'package:sepot/models/contact.dart';
+import 'package:sepot/models/group.dart';
+import 'package:sepot/models/identity.dart';
+import 'package:sepot/providers/contacts_provider.dart';
+import 'package:sepot/providers/conversations_provider.dart';
+import 'package:sepot/providers/groups_provider.dart';
+import 'package:sepot/providers/identity_provider.dart';
+
+/// Profil skupiny podle role: ne-správce nesmí vidět správcovské akce
+/// (odebrat, udělat správcem), jen vysvětlení; správce je dostane v nabídce
+/// člena.
+class _FakeIdentityNotifier extends IdentityNotifier {
+  _FakeIdentityNotifier(this._identity);
+  final LocalIdentity _identity;
+
+  @override
+  Future<LocalIdentity?> build() async => _identity;
+}
+
+class _NoContacts extends ContactsNotifier {
+  @override
+  Future<List<Contact>> build() async => const [];
+}
+
+final _ja = LocalIdentity(
+  displayName: 'Já',
+  identityPrivateKey: Uint8List(32),
+  identityPublicKey: Uint8List(33),
+  createdAt: DateTime.utc(2026, 1, 1),
+);
+
+const _spravce = 'SPRAVCE0001';
+const _clen = 'CLENKA0002';
+
+Group _group({required bool jaSpravce, required bool vsichniV2}) => Group(
+      id: 'g1',
+      name: 'Chata v srpnu',
+      memberIds: [_ja.id, _spravce, _clen],
+      createdBy: _spravce,
+      admins: [_spravce, if (jaSpravce) _ja.id],
+      v2Members: vsichniV2 ? {_spravce, _clen} : {_spravce},
+    );
+
+Widget _app(Group group) => ProviderScope(
+      overrides: [
+        identityProvider.overrideWith(() => _FakeIdentityNotifier(_ja)),
+        contactsProvider.overrideWith(_NoContacts.new),
+        groupProvider.overrideWith((ref, id) async => group),
+        conversationProvider.overrideWith((ref, id) async => null),
+      ],
+      child: MaterialApp(
+        locale: const Locale('cs'),
+        localizationsDelegates: AppLocalizations.localizationsDelegates,
+        supportedLocales: AppLocalizations.supportedLocales,
+        home: GroupProfileScreen(groupId: group.id),
+      ),
+    );
+
+Future<void> _pump(WidgetTester tester, Group group) async {
+  tester.view.physicalSize = const Size(1080, 3200);
+  tester.view.devicePixelRatio = 2.0;
+  addTearDown(tester.view.reset);
+  await tester.pumpWidget(_app(group));
+  await tester.pumpAndSettle();
+}
+
+ProfileAction _addAction(WidgetTester tester) => tester.widget<ProfileAction>(
+    find.widgetWithText(ProfileAction, 'Přidat'));
+
+void main() {
+  testWidgets('ne-správce ve vynucené skupině: akce skryté, vysvětlení',
+      (tester) async {
+    await _pump(tester, _group(jaSpravce: false, vsichniV2: true));
+    expect(tester.takeException(), isNull);
+
+    expect(find.text('Správce'), findsOneWidget,
+        reason: 'odznak u skutečného správce');
+    expect(find.text('Jméno, členy a správce skupiny mění jen správci.'),
+        findsOneWidget);
+    expect(_addAction(tester).onTap, isNull,
+        reason: 'přidávat lidi smí jen správce');
+    expect(find.byIcon(Icons.edit_outlined), findsNothing,
+        reason: 'přejmenování jen správcem');
+
+    await tester.tap(find.textContaining('SEP-CLENKA'));
+    await tester.pumpAndSettle();
+    expect(find.text('Odebrat ze skupiny'), findsNothing);
+    expect(find.text('Udělat správcem'), findsNothing);
+  });
+
+  testWidgets('ne-správce s v1 členem ve skupině smí přidávat, ne spravovat',
+      (tester) async {
+    await _pump(tester, _group(jaSpravce: false, vsichniV2: false));
+    expect(_addAction(tester).onTap, isNotNull);
+    expect(find.textContaining('Přidávat lidi zatím může každý'),
+        findsOneWidget);
+    expect(find.byIcon(Icons.edit_outlined), findsNothing);
+  });
+
+  testWidgets('správce dostane nabídku člena s odebráním a rolí',
+      (tester) async {
+    await _pump(tester, _group(jaSpravce: true, vsichniV2: true));
+    expect(find.text('Správce'), findsNWidgets(2));
+    expect(_addAction(tester).onTap, isNotNull);
+    expect(find.byIcon(Icons.edit_outlined), findsOneWidget);
+    expect(find.textContaining('mění jen správci'), findsNothing);
+
+    await tester.tap(find.textContaining('SEP-CLENKA'));
+    await tester.pumpAndSettle();
+    expect(find.text('Odebrat ze skupiny'), findsOneWidget);
+    expect(find.text('Udělat správcem'), findsOneWidget);
+  });
+}