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/message.rs

@@ -59,6 +59,8 @@ pub enum AppMessage {
     // === Panels (COSMIC-managed) ===
     /// Toggle a context drawer page.
     ToggleContextPage(ContextPage),
+    /// Toggle the nav bar (left panel) visibility.
+    ToggleNavBar,
 
     // === Metadata ===
     /// Refresh metadata from the current document.