feat(theme): shift Android palette from indigo to deep violet to match web identity

lib/screens/briefing/briefing_screen.dart

@@ -462,7 +462,7 @@ class _GradientSendButton extends StatelessWidget {
             : const LinearGradient(
                 begin: Alignment.topLeft,
                 end: Alignment.bottomRight,
-                colors: [Color(0xFF6366F1), Color(0xFF4F46E5)],
+                colors: [Color(0xFF7C3AED), Color(0xFF5B21B6)],
               ),
         color: disabled ? scheme.onSurface.withValues(alpha: 0.12) : null,
         borderRadius: BorderRadius.circular(10),

lib/screens/calendar/event_form_sheet.dart

@@ -57,7 +57,7 @@ class _EventFormSheetState extends ConsumerState<EventFormSheet> {
     '#EF4444',
     '#F59E0B',
     '#10B981',
-    '#6366F1',
+    '#7C3AED',
     '#8B5CF6',
     '#EC4899',
   ];

lib/screens/library/project_tasks_screen.dart

@@ -55,11 +55,11 @@ class _ProjectTasksScreenState extends ConsumerState<ProjectTasksScreen> {
   }
 
   Color _parseColor(String? hex) {
-    if (hex == null || hex.isEmpty) return const Color(0xFF6366F1);
+    if (hex == null || hex.isEmpty) return const Color(0xFF7C3AED);
     try {
       return Color(int.parse(hex.replaceFirst('#', '0xFF')));
     } catch (_) {
-      return const Color(0xFF6366F1);
+      return const Color(0xFF7C3AED);
     }
   }