refactor: Update placeholder color in chat input for improved visibility

- Changed the placeholder color in the modern chat input to use the input text color with adjusted alpha for better contrast and visibility in both light and dark themes.
- This enhancement aims to improve user experience by ensuring the placeholder text is more legible across different brightness settings.

lib/features/chat/widgets/modern_chat_input.dart

@@ -649,7 +649,9 @@ class _ModernChatInputState extends ConsumerState<ModernChatInput>
     final Color composerBackground = brightness == Brightness.dark
         ? composerSurface.withValues(alpha: 0.78)
         : context.conduitTheme.surfaceContainerHighest;
-    final Color placeholderBase = context.conduitTheme.inputPlaceholder;
+    final Color placeholderBase = context.conduitTheme.inputText.withValues(
+      alpha: 0.64,
+    );
     final Color placeholderFocused = context.conduitTheme.inputText.withValues(
       alpha: 0.64,
     );