feat: add composer autofocus management for improved chat input experience

- Introduced ComposerAutofocusEnabled provider to manage the auto-focus state of the chat composer, allowing for better control over user interactions.
- Updated ModernChatInput to respect the autofocus setting, ensuring the keyboard behavior aligns with user intent and context.
- Enhanced ChatPage to suppress auto-focus when opening the slide drawer, improving user experience during navigation.
- Refactored SlideDrawer to include an onOpenStart callback for dismissing the keyboard, ensuring a smoother transition when the drawer is opened.

lib/features/chat/providers/chat_providers.dart

@@ -75,6 +75,16 @@ class ComposerHasFocus extends _$ComposerHasFocus {
   void set(bool value) => state = value;
 }
 
+// Whether the chat composer is allowed to auto-focus.
+// When false, the composer will remain unfocused until the user taps it.
+@Riverpod(keepAlive: true)
+class ComposerAutofocusEnabled extends _$ComposerAutofocusEnabled {
+  @override
+  bool build() => true;
+
+  void set(bool value) => state = value;
+}
+
 // Chat messages notifier class
 class ChatMessagesNotifier extends Notifier<List<ChatMessage>> {
   StreamingResponseController? _messageStream;