feat(theme): add tool_output color to themes

- Added a `tool_output` color to the `Theme` struct.
- Updated all built-in themes to include the new color.
- Modified the TUI to use the `tool_output` color for rendering tool output.

crates/owlen-core/src/formatting.rs

@@ -91,6 +91,11 @@ impl MessageFormatter {
             Some(thinking)
         };
 
+        // If the result is empty but we have thinking content, show a placeholder
+        if result.trim().is_empty() && thinking_result.is_some() {
+            result.push_str("[Thinking...]");
+        }
+
         (result, thinking_result)
     }
 }