fix: warm sheet header, welcome state for AI chat, consistent colors

- Custom sheet header with warm canvas background (no white bar)
- Tab picker styled as warm pills (not default segmented control)
- AI Chat welcome state: sparkles icon + 'What did you eat?' prompt
- All backgrounds use canvas color consistently

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/Features/Assistant/AssistantChatView.swift

@@ -10,6 +10,27 @@ struct AssistantChatView: View {
             ScrollViewReader { proxy in
                 ScrollView {
                     LazyVStack(spacing: 8) {
+                        // Welcome state when no messages
+                        if vm.messages.isEmpty {
+                            VStack(spacing: 16) {
+                                Spacer(minLength: 60)
+                                Image(systemName: "sparkles")
+                                    .font(.system(size: 40))
+                                    .foregroundStyle(Color.accentWarm.opacity(0.4))
+                                Text("What did you eat?")
+                                    .font(.title3.weight(.semibold))
+                                    .foregroundStyle(Color.textPrimary)
+                                Text("Describe your food naturally.\n\"2 eggs and toast for breakfast\"\nor snap a photo of your meal.")
+                                    .font(.subheadline)
+                                    .foregroundStyle(Color.textSecondary)
+                                    .multilineTextAlignment(.center)
+                                    .lineSpacing(4)
+                                Spacer(minLength: 60)
+                            }
+                            .frame(maxWidth: .infinity)
+                            .padding(.horizontal, 32)
+                        }
+
                         ForEach(vm.messages) { message in
                             chatBubble(message)
                                 .id(message.id)