feat(ui): mobile UI v2 — contrast, avatars, room-list density, compose FAB, gradient login (v1.8.0+12)

From a 7-agent design workflow (3 lenses judged + synthesised). Shipped the
high-impact/low-risk tier:
- message bubbles: self #1265ED (WCAG AA pass), incoming #1C2452 + hairline
- deterministic per-user avatar colours (8-hue brand palette)
- room tile: brighter unread preview, compact density, aligned timestamp
- FAB -> New message (thumb zone), explore -> AppBar
- login + welcome brand gradient, card outlines
- input maxLines cap, bubble max-width viewport fraction, AppBar hairline

All verified on production build in headless Chromium at mobile size.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/features/chat/presentation/message_bubble.dart

@@ -1,10 +1,11 @@
-// Version: 1.3.1 | Created: 2026-04-01 | Updated: 2026-07-03
+// Version: 1.4.0 | Created: 2026-04-01 | Updated: 2026-07-04
 // Message bubble widget. Handles text, images, files, redacted, replies.
 
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
+import '../../../app/theme.dart';
 import '../../../shared/widgets/matrix_avatar.dart';
 import '../domain/message_model.dart';
 
@@ -79,14 +80,19 @@ class _BubbleContent extends StatelessWidget {
     final theme = Theme.of(context);
 
     final bgColour = isMine
-        ? theme.colorScheme.primary
-        : theme.colorScheme.surfaceContainerHighest;
+        ? M8Colours.selfBubble
+        : M8Colours.darkSurfaceVariant;
     final textColour = isMine ? Colors.white : theme.colorScheme.onSurface;
+    final maxBubbleWidth =
+        (MediaQuery.sizeOf(context).width.clamp(0, 560) * 0.78).toDouble();
 
     return Container(
-      constraints: const BoxConstraints(maxWidth: 320),
+      constraints: BoxConstraints(maxWidth: maxBubbleWidth),
       decoration: BoxDecoration(
         color: bgColour,
+        border: isMine
+            ? null
+            : Border.all(color: theme.colorScheme.outline.withAlpha(31)),
         borderRadius: BorderRadius.only(
           topLeft: const Radius.circular(16),
           topRight: const Radius.circular(16),