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/config.rs

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 // src/config.rs
+//
+// Global configuration for the application with cosmic-config support.
 
 use cosmic::cosmic_config::{self, CosmicConfigEntry, cosmic_config_derive::CosmicConfigEntry};
 use std::path::PathBuf;
@@ -10,6 +12,10 @@ use std::path::PathBuf;
 pub struct AppConfig {
     /// Optional default directory to open images from.
     pub default_image_dir: Option<PathBuf>,
+    /// Whether the nav bar (left panel) is visible.
+    pub nav_bar_visible: bool,
+    /// Whether the context drawer (right panel) is visible.
+    pub context_drawer_visible: bool,
 }
 
 impl Default for AppConfig {
@@ -17,6 +23,8 @@ impl Default for AppConfig {
         Self {
             // TODO: Use xdg dir for picture
             default_image_dir: Some(PathBuf::from("~/Pictures")),
+            nav_bar_visible: false,
+            context_drawer_visible: false,
         }
     }
 }