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  can be changed or withdrawn; older clients see them as text.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

@@ -0,0 +1,83 @@
+import 'dart:ui';
+
+import 'package:flutter/material.dart';
+import 'package:sepot/core/theme/app_theme.dart';
+
+/// Průsvitná plocha s rozmazáním obsahu pod sebou (hlavička chatu, pole pro
+/// psaní, spodní navigace). Blur je drahý, proto jen na pár plovoucích
+/// lištách, nikdy na položkách seznamu.
+class GlassSurface extends StatelessWidget {
+  const GlassSurface({
+    super.key,
+    required this.child,
+    this.borderRadius = BorderRadius.zero,
+    this.padding = EdgeInsets.zero,
+    this.border = true,
+  });
+
+  final Widget child;
+  final BorderRadius borderRadius;
+  final EdgeInsets padding;
+  final bool border;
+
+  @override
+  Widget build(BuildContext context) {
+    return ClipRRect(
+      borderRadius: borderRadius,
+      child: BackdropFilter(
+        filter: ImageFilter.blur(sigmaX: 18, sigmaY: 18),
+        child: Container(
+          padding: padding,
+          decoration: BoxDecoration(
+            color: SepotColors.glass,
+            borderRadius: borderRadius,
+            border: border
+                ? Border.all(color: SepotColors.glassBorder)
+                : null,
+          ),
+          child: child,
+        ),
+      ),
+    );
+  }
+}
+
+/// Tečková tapeta pod konverzací — dává průsvitným lištám co rozmazat
+/// a chatu hloubku, aniž by rušila text.
+class ChatWallpaper extends StatelessWidget {
+  const ChatWallpaper({super.key, required this.child});
+
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return CustomPaint(
+      painter: const _DotsPainter(),
+      isComplex: true,
+      willChange: false,
+      child: child,
+    );
+  }
+}
+
+class _DotsPainter extends CustomPainter {
+  const _DotsPainter();
+
+  static const _spacing = 18.0;
+  static const _radius = 1.0;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    canvas.drawRect(
+        Offset.zero & size, Paint()..color = SepotColors.background);
+    final dot = Paint()..color = SepotColors.mist.withValues(alpha: 0.11);
+    for (var y = _spacing / 2; y < size.height; y += _spacing) {
+      for (var x = _spacing / 2; x < size.width; x += _spacing) {
+        canvas.drawCircle(Offset(x, y), _radius, dot);
+      }
+    }
+  }
+
+  @override
+  bool shouldRepaint(covariant _DotsPainter oldDelegate) => false;
+}