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/main.rs

@@ -3,6 +3,7 @@
 
 mod app;
 mod config;
+mod constant;
 mod i18n;
 
 use anyhow::Result;