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/auth/presentation/login_screen.dart

@@ -1,4 +1,4 @@
-// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-03
+// Version: 1.3.0 | Created: 2026-04-01 | Updated: 2026-07-04
 // Login screen. Username + password only. No registration link.
 // Respects system theme preference.
 
@@ -55,7 +55,15 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
     final theme = Theme.of(context);
 
     return Scaffold(
-      body: SafeArea(
+      body: DecoratedBox(
+        decoration: const BoxDecoration(
+          gradient: LinearGradient(
+            begin: Alignment.topCenter,
+            end: Alignment.bottomCenter,
+            colors: [Color(0xFF0A0F2E), Color(0xFF131A42)],
+          ),
+        ),
+        child: SafeArea(
         child: Center(
           child: SingleChildScrollView(
             padding: const EdgeInsets.symmetric(horizontal: 32, vertical: 24),
@@ -89,6 +97,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
             ),
           ),
         ),
+        ),
       ),
     );
   }