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_input.dart

@@ -1,4 +1,4 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.1.1 | Created: 2026-04-01 | Updated: 2026-07-04
 // Message input bar — text, send, attach file, reply quote.
 // File picker enabled in Phase 2 via file_picker ^8.0.0.
 
@@ -195,7 +195,8 @@ class _MessageInputState extends State<MessageInput> {
                       ),
                       textInputAction: TextInputAction.send,
                       onSubmitted: (_) => _submit(),
-                      maxLines: null,
+                      minLines: 1,
+                      maxLines: 6,
                       keyboardType: TextInputType.multiline,
                     ),
                   ),