feat: upgrade marked to v17 with marked-highlight extension

- Update marked from ^16.2.1 to ^17.0.3
- Add marked-highlight ^2.2.3 for syntax highlighting support
- Replace deprecated marked.setOptions() with marked.use() in renderer.js
- Extract highlight config into markedHighlight() extension (required in v17)
- Update test mock to reflect new API (use instead of setOptions)

tests/setup.js

@@ -59,7 +59,7 @@ global.window.electronAPI = {
 // Mock marked library
 global.window.marked = {
   parse: jest.fn((text) => `<p>${text}</p>`),
-  setOptions: jest.fn()
+  use: jest.fn()
 };
 
 // Mock DOMPurify