feat(ui): add configurable role label display and syntax highlighting support

- Introduce `RoleLabelDisplay` enum (inline, above, none) and integrate it into UI rendering and message formatting.
- Replace `show_role_labels` boolean with `role_label_mode` across config, formatter, session, and TUI components.
- Add `syntax_highlighting` boolean to UI settings with default `false` and support in message rendering.
- Update configuration schema version to 1.3.0 and provide deserialization handling for legacy boolean values.
- Extend theme definitions with code block styling fields (background, border, text, keyword, string, comment) and default values in `Theme`.
- Adjust related modules (`formatting.rs`, `ui.rs`, `session.rs`, `chat_app.rs`) to use the new settings and theme fields.

crates/owlen-core/src/formatting.rs

@@ -1,19 +1,20 @@
 use crate::types::Message;
+use crate::ui::RoleLabelDisplay;
 
 /// Formats messages for display across different clients.
 #[derive(Debug, Clone)]
 pub struct MessageFormatter {
     wrap_width: usize,
-    show_role_labels: bool,
+    role_label_mode: RoleLabelDisplay,
     preserve_empty_lines: bool,
 }
 
 impl MessageFormatter {
     /// Create a new formatter
-    pub fn new(wrap_width: usize, show_role_labels: bool) -> Self {
+    pub fn new(wrap_width: usize, role_label_mode: RoleLabelDisplay) -> Self {
         Self {
             wrap_width: wrap_width.max(20),
-            show_role_labels,
+            role_label_mode,
             preserve_empty_lines: false,
         }
     }
@@ -29,9 +30,19 @@ impl MessageFormatter {
         self.wrap_width = width.max(20);
     }
 
-    /// Whether role labels should be shown alongside messages
+    /// The configured role label layout preference.
+    pub fn role_label_mode(&self) -> RoleLabelDisplay {
+        self.role_label_mode
+    }
+
+    /// Whether any role label should be shown alongside messages.
     pub fn show_role_labels(&self) -> bool {
-        self.show_role_labels
+        !matches!(self.role_label_mode, RoleLabelDisplay::None)
+    }
+
+    /// Update the role label layout preference.
+    pub fn set_role_label_mode(&mut self, mode: RoleLabelDisplay) {
+        self.role_label_mode = mode;
     }
 
     pub fn format_message(&self, message: &Message) -> Vec<String> {