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/rooms/presentation/room_tile.dart

@@ -1,9 +1,10 @@
-// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-02
-// Individual room list tile. Kept under 100 lines.
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-07-04
+// Individual room list tile. Kept under 120 lines.
 
 import 'package:flutter/material.dart';
 import 'package:timeago/timeago.dart' as timeago;
 
+import '../../../app/theme.dart';
 import '../../../shared/widgets/matrix_avatar.dart';
 import '../domain/room_model.dart';
 
@@ -19,6 +20,8 @@ class RoomTile extends StatelessWidget {
     final hasUnread = room.unreadCount > 0;
 
     return ListTile(
+      visualDensity: VisualDensity.compact,
+      contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 4),
       leading: MatrixAvatar(
         name: room.displayName,
         avatarUrl: room.avatarUrl,
@@ -38,12 +41,15 @@ class RoomTile extends StatelessWidget {
               maxLines: 1,
               overflow: TextOverflow.ellipsis,
               style: theme.textTheme.bodySmall?.copyWith(
-                color: theme.colorScheme.onSurface.withAlpha(153),
+                color: hasUnread
+                    ? theme.colorScheme.onSurface.withAlpha(230)
+                    : M8Colours.subtleText,
+                fontWeight: hasUnread ? FontWeight.w500 : FontWeight.normal,
               ),
             )
           : null,
       trailing: Column(
-        mainAxisAlignment: MainAxisAlignment.center,
+        mainAxisAlignment: MainAxisAlignment.spaceBetween,
         crossAxisAlignment: CrossAxisAlignment.end,
         children: [
           if (room.lastActivityAt != null)
@@ -52,13 +58,14 @@ class RoomTile extends StatelessWidget {
               style: theme.textTheme.labelSmall?.copyWith(
                 color: hasUnread
                     ? theme.colorScheme.primary
-                    : theme.colorScheme.onSurface.withAlpha(102),
+                    : M8Colours.subtleText,
               ),
             ),
           if (hasUnread) ...[
             const SizedBox(height: 4),
             _UnreadBadge(count: room.unreadCount),
-          ],
+          ] else
+            const SizedBox(height: 8),
         ],
       ),
       onTap: onTap,