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

README.md

@@ -4,6 +4,52 @@ An image viewer application for the COSMIC™ desktop
 
 ![Screenshot](docs/images/screenshot.png)
 
+## Features
+
+- **Multi-format support**: Raster images (PNG, JPEG, WebP, etc.), SVG vector graphics, and PDF documents
+- **Navigation**: Browse through folders with keyboard shortcuts
+- **Transformations**: Rotate, flip, and crop images
+- **Zoom & Pan**: Flexible viewing with zoom controls and panning
+- **Multi-page documents**: Navigate PDF pages with thumbnail previews
+- **Metadata display**: View EXIF data and file information
+- **Wallpaper setting**: Set images as desktop wallpaper (multi-DE support)
+
+## Architecture
+
+Noctua follows Clean Architecture principles with clear separation of concerns:
+
+```
+src/
+├── main.rs              # Application entry point
+├── ui/                  # UI Layer (COSMIC interface)
+│   ├── app.rs           # Application state & lifecycle
+│   ├── model.rs         # UI state + cached render data
+│   ├── update.rs        # Message handlers
+│   ├── sync.rs          # Model synchronization
+│   ├── views/           # View components
+│   └── components/      # Reusable widgets
+├── application/         # Application Layer (use cases)
+│   ├── document_manager.rs  # Document orchestration
+│   ├── commands/        # Write operations (Transform, Crop)
+│   └── services/        # Shared services (Cache)
+├── domain/              # Domain Layer (business logic)
+│   ├── document/        # Document abstractions & operations
+│   │   ├── core/        # Traits & types (Renderable, Transformable)
+│   │   ├── types/       # Implementations (Raster, Vector, Portable)
+│   │   └── operations/  # Transform, render, export operations
+│   └── errors.rs        # Domain errors
+└── infrastructure/      # Infrastructure Layer (external systems)
+    ├── loaders/         # Document loading (image, SVG, PDF)
+    ├── cache/           # Thumbnail caching
+    ├── filesystem/      # File operations
+    └── system/          # System integration (wallpaper)
+```
+
+**Key Patterns:**
+- **MVU (Model-View-Update)**: Elm architecture via libcosmic
+- **Command Pattern**: All operations go through commands
+- **Dependency Inversion**: Domain has no dependencies on infrastructure
+- **Type-Erased Documents**: `DocumentContent` enum for unified handling
 
 ## Installation