feature: PDF and PDF thumbnails and refresh UI

- Implement PDF and PDF thumbnail generation with incremental loading
- Add UI refresh mechanism (tick counter + RefreshView message)
- Improve fl! macro with named parameters
- Clean up code organization (mod.rs: wiring, model.rs: state only)

src/app/message.rs

@@ -1,88 +1,71 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 // src/app/message.rs
 //
-// All application messages (events, user actions, signals).
+// Application messages: events, user actions, and internal signals.
 
 use std::path::PathBuf;
 
 use crate::app::ContextPage;
 
-/// Messages emitted by user actions, async I/O, or internal signals.
 #[derive(Debug, Clone)]
 pub enum AppMessage {
-    // === File / Navigation ===
-    /// Open a file at the given path.
+    // File / navigation.
     #[allow(dead_code)]
     OpenPath(PathBuf),
-    /// Navigate to the next document in folder.
     NextDocument,
-    /// Navigate to the previous document in folder.
     PrevDocument,
+    GotoPage(u32),
+    GenerateThumbnailPage(u32),
 
-    // === Transformations ===
-    /// Rotate 90° clockwise.
+    // Transformations.
     RotateCW,
-    /// Rotate 90° counter-clockwise.
     RotateCCW,
-    /// Flip horizontally (mirror).
     FlipHorizontal,
-    /// Flip vertically.
     FlipVertical,
 
-    // === Zoom ===
-    /// Zoom in by a fixed step.
+    // View / zoom.
     ZoomIn,
-    /// Zoom out by a fixed step.
     ZoomOut,
-    /// Reset zoom to 100%.
     ZoomReset,
-    /// Fit document to window.
     ZoomFit,
-    /// Update zoom and pan from viewer (mouse interaction).
-    ViewerStateChanged { scale: f32, offset_x: f32, offset_y: f32 },
+    ViewerStateChanged {
+        scale: f32,
+        offset_x: f32,
+        offset_y: f32,
+    },
 
-    // === Pan ===
-    /// Pan image left.
+    // Pan control.
     PanLeft,
-    /// Pan image right.
     PanRight,
-    /// Pan image up.
     PanUp,
-    /// Pan image down.
     PanDown,
-    /// Reset pan to center.
     PanReset,
 
-    // === Tool Modes ===
-    /// Toggle crop mode.
+    // Tool modes.
     ToggleCropMode,
-    /// Toggle scale mode.
     ToggleScaleMode,
 
-    // === Panels (COSMIC-managed) ===
-    /// Toggle a context drawer page.
+    // Panels.
     ToggleContextPage(ContextPage),
-    /// Toggle the nav bar (left panel) visibility.
     ToggleNavBar,
 
-    // === Metadata ===
-    /// Refresh metadata from the current document.
+    // Metadata.
     #[allow(dead_code)]
     RefreshMetadata,
 
-    // === Wallpaper ===
-    /// Set current image as wallpaper.
+    // Wallpaper.
     SetAsWallpaper,
 
-    // === Errors ===
-    /// Display an error message.
+    // Errors.
     #[allow(dead_code)]
     ShowError(String),
-    /// Clear the current error.
     #[allow(dead_code)]
     ClearError,
 
-    /// Fallback for unhandled or no-op cases.
+    // UI refresh.
+    RefreshView,
+
+    // Fallback.
     #[allow(dead_code)]
     NoOp,
 }