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/core/config/app_config.dart

@@ -11,7 +11,7 @@ abstract final class AppConfig {
     'turns:matrix.m8chat.au:5349',
   ];
   static const String appName = 'M8Chat';
-  static const String appVersion = '1.7.0';
+  static const String appVersion = '1.8.0';
 
   // Jitsi conferencing
   static const String jitsiDomain = 'conf.m8chat.au';