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  can be changed or withdrawn; older clients see them as text.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/core/services/avatar_image.dart

@@ -0,0 +1,38 @@
+import 'dart:isolate';
+import 'dart:math' as math;
+import 'dart:typed_data';
+
+import 'package:image/image.dart' as img;
+import 'package:sepot/core/services/image_processor.dart';
+
+/// Strana profilové fotky v pixelech — víc se v appce nikde nezobrazí.
+const avatarSide = 256;
+
+/// Vybranou fotku otočí podle EXIF, ořízne na středový čtverec, zmenší
+/// a znovu zakóduje do JPEG bez metadat (EXIF, GPS, model telefonu).
+/// Null = obrázek nejde přečíst.
+Future<Uint8List?> prepareAvatarJpeg(Uint8List input) => Isolate.run(() {
+      final img.Image? decoded;
+      try {
+        decoded = img.decodeImage(input);
+      } on Object {
+        // Dekodéry na poškozených datech háží místo null.
+        return null;
+      }
+      if (decoded == null) return null;
+      final upright = img.bakeOrientation(decoded);
+      final side = math.min(upright.width, upright.height);
+      final square = img.copyCrop(
+        upright,
+        x: (upright.width - side) ~/ 2,
+        y: (upright.height - side) ~/ 2,
+        width: side,
+        height: side,
+      );
+      final small = img.copyResize(square,
+          width: avatarSide,
+          height: avatarSide,
+          interpolation: img.Interpolation.average);
+      ImageProcessor.stripMetadata(small);
+      return Uint8List.fromList(img.encodeJpg(small, quality: 85));
+    });