Display backend error messages in red (#18)

Fixed Greptile review feedback:
- Kept system_message() using TEXT_MUTED (gray) for informational messages
- Added new error_message() style using ERROR (red) with italic for backend errors
- Error detection in rendering.rs already handles displaying errors in red

* style(ui): change system message color to red for error visibility

* fix: keep system_message() muted, add separate error_message() style

Address Greptile review feedback on PR #18:
- Reverted system_message() to use TEXT_MUTED (gray) for informational messages
- Added new error_message() style with ERROR (red) + italic for backend errors
- Rendering logic in rendering.rs already detects errors and uses colors.error

This ensures regular system messages (like 'Resumed session') stay muted gray
while actual error messages are displayed in red.

Author: echobt

src/cortex-core/src/style.rs

@@ -362,14 +362,20 @@ impl CortexStyle {
         Style::default().fg(SKY_BLUE)
     }
 
-    /// System message style: muted italic text
+    /// System message style: muted italic text for informational system messages
     #[inline]
     pub fn system_message() -> Style {
         Style::default()
             .fg(TEXT_MUTED)
             .add_modifier(Modifier::ITALIC)
     }
 
+    /// Error message style: red italic text for backend error messages
+    #[inline]
+    pub fn error_message() -> Style {
+        Style::default().fg(ERROR).add_modifier(Modifier::ITALIC)
+    }
+
     /// Code style: electric blue text on surface background
     #[inline]
     pub fn code() -> Style {
@@ -530,6 +536,7 @@ mod tests {
         let _ = CortexStyle::user_message();
         let _ = CortexStyle::assistant_message();
         let _ = CortexStyle::system_message();
+        let _ = CortexStyle::error_message();
         let _ = CortexStyle::code();
         let _ = CortexStyle::border();
         let _ = CortexStyle::border_focused();