feat: brand retheme to m8chat.au blues, fix black logo, v1.6.0+8

- m8logo.svg: inline fills (flutter_svg ignores <style> blocks; logo rendered black)
- theme.dart v2.0.0: purple -> site palette (#1265ED / #3B8BFF / navy darks)
- app_config.dart: appVersion 1.3.0 -> 1.6.0 (profile screen showed stale version)
- manifest.json: theme/background colours to match
- Remove em-dashes from user-visible UI strings (global style rule)
- chat_screen.dart: fix use_build_context_synchronously in _leaveRoom
- Deployed to app2.m8chat.au after full functionality audit (all green)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/core/config/app_config.dart

@@ -1,4 +1,4 @@
-// Version: 2.0.0 | Created: 2026-04-01 | Updated: 2026-04-11
+// Version: 2.1.0 | Created: 2026-04-01 | Updated: 2026-07-03
 // App-wide constants. Change matrixBaseUrl for different environments.
 
 abstract final class AppConfig {
@@ -11,7 +11,7 @@ abstract final class AppConfig {
     'turns:matrix.m8chat.au:5349',
   ];
   static const String appName = 'M8Chat';
-  static const String appVersion = '1.3.0';
+  static const String appVersion = '1.6.0';
 
   // Jitsi conferencing
   static const String jitsiDomain = 'conf.m8chat.au';