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/rooms_screen.dart

@@ -1,4 +1,4 @@
-// Version: 1.6.0 | Created: 2026-04-01 | Updated: 2026-07-04
+// Version: 1.7.0 | Created: 2026-04-01 | Updated: 2026-07-04
 // Main rooms list screen with bottom navigation.
 
 import 'package:flutter/material.dart';
@@ -7,6 +7,7 @@ import 'package:go_router/go_router.dart';
 
 import 'dart:async';
 
+import '../../../app/theme.dart';
 import '../../../core/network/matrix_client.dart';
 import '../../help/presentation/help_tab.dart';
 import '../../panic/presentation/panic_button.dart';
@@ -123,19 +124,20 @@ class _RoomsScreenState extends ConsumerState<RoomsScreen> {
                 ),
                 if (!_searchActive)
                   IconButton(
-                    icon: const Icon(Icons.edit_square),
-                    tooltip: 'New message',
-                    onPressed: () => showUserSearchDialog(context),
+                    icon: const Icon(Icons.explore_outlined),
+                    tooltip: 'Find rooms',
+                    onPressed: () => showPublicRoomsDialog(context),
                   ),
               ],
             )
           : null,
       body: _buildBody(),
       floatingActionButton: _selectedIndex == 0 && !_searchActive
-          ? FloatingActionButton(
-              onPressed: () => showPublicRoomsDialog(context),
-              tooltip: 'Find rooms',
-              child: const Icon(Icons.explore_outlined),
+          ? FloatingActionButton.extended(
+              onPressed: () => showUserSearchDialog(context),
+              tooltip: 'New message',
+              icon: const Icon(Icons.edit_square),
+              label: const Text('New message'),
             )
           : null,
       bottomNavigationBar: NavigationBar(
@@ -253,16 +255,16 @@ class _EmptyRoomsState extends StatelessWidget {
             Text(
               'No rooms yet',
               style: theme.textTheme.titleMedium?.copyWith(
-                color: theme.colorScheme.onSurface.withAlpha(153),
+                color: M8Colours.subtleText,
               ),
             ),
             const SizedBox(height: 8),
             Text(
-              'Use the pencil icon above to message someone, or find '
+              'Tap the New message button to message someone, or find '
               'a public room to join.',
               textAlign: TextAlign.center,
               style: theme.textTheme.bodySmall?.copyWith(
-                color: theme.colorScheme.onSurface.withAlpha(102),
+                color: M8Colours.subtleText,
               ),
             ),
             const SizedBox(height: 20),