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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,38 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:sepot/providers/storage_provider.dart';
+
+/// Rozepsané zprávy (contactId → text). Jeden záznam v šifrovaném nastavení,
+/// ne pole konverzace: konverzace se ukládá celá jako load → změna → save
+/// a zápis konceptu při psaní by mohl přepsat zprávu, která mezitím dorazila.
+class DraftsNotifier extends AsyncNotifier<Map<String, String>> {
+  static const key = 'drafts';
+
+  @override
+  Future<Map<String, String>> build() async {
+    final raw =
+        await ref.read(storageProvider).getSetting<Map<String, dynamic>>(key);
+    return {
+      for (final e in (raw ?? const {}).entries)
+        if (e.value is String) e.key: e.value as String,
+    };
+  }
+
+  Future<void> set(String contactId, String text) async {
+    // Počkat na načtení — jinak by zápis před dokončením build() smazal
+    // koncepty ostatních chatů.
+    final current = {...await future};
+    final trimmed = text.trim();
+    if ((current[contactId] ?? '') == trimmed) return;
+    if (trimmed.isEmpty) {
+      current.remove(contactId);
+    } else {
+      current[contactId] = trimmed;
+    }
+    state = AsyncData(current);
+    await ref.read(storageProvider).setSetting(key, current);
+  }
+}
+
+final draftsProvider =
+    AsyncNotifierProvider<DraftsNotifier, Map<String, String>>(
+        DraftsNotifier.new);