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/view/mod.rs

@@ -5,6 +5,7 @@
 
 mod canvas;
 pub mod footer;
+pub mod header;
 pub mod panels;
 
 use cosmic::Element;