feat(tui): add markdown rendering support and toggle command

- Introduce new `owlen-markdown` crate that converts Markdown strings to `ratatui::Text` with headings, lists, bold/italic, and inline code.
- Add `render_markdown` config option (default true) and expose it via `app.render_markdown_enabled()`.
- Implement `:markdown [on|off]` command to toggle markdown rendering.
- Update help overlay to document the new markdown toggle.
- Adjust UI rendering to conditionally apply markdown styling based on the markdown flag and code mode.
- Wire the new crate into `owlen-tui` Cargo.toml.

crates/owlen-core/src/mcp/permission.rs

@@ -156,13 +156,14 @@ mod tests {
     use super::*;
     use crate::mcp::LocalMcpClient;
     use crate::tools::registry::ToolRegistry;
+    use crate::ui::NoOpUiController;
     use crate::validation::SchemaValidator;
     use std::sync::atomic::{AtomicBool, Ordering};
 
     #[tokio::test]
     async fn test_permission_layer_filters_dangerous_tools() {
         let config = Arc::new(Config::default());
-        let ui = Arc::new(crate::ui::NoOpUiController);
+        let ui = Arc::new(NoOpUiController);
         let registry = Arc::new(ToolRegistry::new(
             Arc::new(tokio::sync::Mutex::new((*config).clone())),
             ui,
@@ -186,7 +187,7 @@ mod tests {
     #[tokio::test]
     async fn test_consent_callback_is_invoked() {
         let config = Arc::new(Config::default());
-        let ui = Arc::new(crate::ui::NoOpUiController);
+        let ui = Arc::new(NoOpUiController);
         let registry = Arc::new(ToolRegistry::new(
             Arc::new(tokio::sync::Mutex::new((*config).clone())),
             ui,