Complete Clean Architecture migration

Phase 1-7: Full migration from src/app/ to Clean Architecture

BREAKING CHANGES:
- Removed src/app/ (old TEA-style implementation)
- Removed src/constant.rs (constants now local to modules)
- Removed deprecated canvas_to_image_coords functions

NEW STRUCTURE:
- src/ui/           - UI Layer (COSMIC interface)
- src/application/  - Application Layer (DocumentManager, Commands)
- src/domain/       - Domain Layer (Document types, Operations)
- src/infrastructure/ - Infrastructure Layer (Loaders, Cache, System)

FEATURES:
- DocumentManager as Single Source of Truth
- Command Pattern for all operations
- Model caching for render data (performance)
- Sync mechanism between DocumentManager and UI Model
- Wallpaper support (COSMIC, KDE, GNOME, feh)
- Thumbnail cache with disk persistence

IMPROVEMENTS:
- Warnings: 62 → 43 (-31%)
- Deprecated warnings: 2 → 0 (-100%)
- Code removed: src/app/ (~2000 lines), constant.rs, deprecated functions
- Better Locality of Reference (constants local to modules)
- Clean separation of concerns
- No circular dependencies

DOCUMENTATION:
- Updated AGENTS.md (100% migration status)
- Updated README.md (architecture section)
- Updated Workflow.md
- Added Migration-Plan.md with full completion summary

TESTS:
- All 41 tests passing
- Build successful (0 errors, 43 warnings)
- Release build verified

Migration Status: ✅ 100% Complete

src/main.rs

@@ -3,15 +3,18 @@
 //
 // Application entry point.
 
-mod app;
+mod ui;
+mod application;
+mod domain;
+mod infrastructure;
+
 mod config;
-mod constant;
 mod i18n;
 
 use anyhow::Result;
 use clap::Parser;
 use cosmic::app::Settings;
-use crate::app::Noctua;
+use crate::ui::NoctuaApp;
 
 #[derive(Parser, Debug, Clone)]
 #[command(version, about)]
@@ -35,6 +38,6 @@ fn main() -> Result<()> {
     env_logger::init();
     let args = Args::parse();
 
-    cosmic::app::run::<Noctua>(Settings::default(), app::Flags::Args(args))
+    cosmic::app::run::<NoctuaApp>(Settings::default(), ui::app::Flags::Args(args))
         .map_err(|e| anyhow::anyhow!(e))
 }