feat(ui): add header toolbar with navigation and transform buttons

- Add header bar with nav toggle, prev/next, rotate and flip buttons
- Extract header rendering to view/header.rs (MVU architecture)
- Add RotateCW, RotateCCW, FlipHorizontal, FlipVertical messages
- Add PrevDocument, NextDocument navigation messages
- Persist nav_bar_visible and context_drawer_visible in config
- Update properties panel with document info display"

src/app/update.rs

@@ -111,6 +111,10 @@ pub fn update(model: &mut AppModel, msg: AppMessage) {
             // Handled in Noctua::update() directly.
         }
 
+        AppMessage::ToggleNavBar => {
+            // Handled in Noctua::update() directly.
+        }
+
         AppMessage::NoOp => {
             // Intentionally do nothing.
         }