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/ui/message.rs

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/app/message.rs
+//
+// Application messages: events, user actions, and internal signals.
+
+use std::path::PathBuf;
+
+use crate::ui::components::crop::DragHandle;
+
+#[derive(Debug, Clone)]
+pub enum AppMessage {
+    // File / navigation.
+    #[allow(dead_code)]
+    OpenPath(PathBuf),
+    NextDocument,
+    PrevDocument,
+    GotoPage(usize),
+    GenerateThumbnailPage(usize),
+
+    // Transformations.
+    RotateCW,
+    RotateCCW,
+    FlipHorizontal,
+    FlipVertical,
+
+    // View / zoom.
+    ZoomIn,
+    ZoomOut,
+    ZoomReset,
+    ZoomFit,
+    ViewerStateChanged {
+        scale: f32,
+        offset_x: f32,
+        offset_y: f32,
+        canvas_size: cosmic::iced::Size,
+        image_size: cosmic::iced::Size,
+    },
+
+    // Pan control.
+    PanLeft,
+    PanRight,
+    PanUp,
+    PanDown,
+    PanReset,
+
+    // Tool modes.
+    ToggleCropMode,
+    ToggleScaleMode,
+
+    // Crop operations.
+    StartCrop,
+    CancelCrop,
+    ApplyCrop,
+    CropDragStart {
+        x: f32,
+        y: f32,
+        handle: DragHandle,
+    },
+    CropDragMove {
+        x: f32,
+        y: f32,
+        max_x: f32,
+        max_y: f32,
+    },
+    CropDragEnd,
+
+    // Panels.
+    ToggleContextPage(crate::ui::app::ContextPage),
+    ToggleNavBar,
+    OpenFormatPanel,
+
+    // Menu.
+    ToggleMainMenu,
+
+    // Format operations.
+    SetPaperFormat(super::model::PaperFormat),
+    SetOrientation(super::model::Orientation),
+
+    // Metadata.
+    #[allow(dead_code)]
+    RefreshMetadata,
+
+    // Save operations.
+    SaveAs,
+
+    // Wallpaper.
+    SetAsWallpaper,
+
+    // Errors.
+    #[allow(dead_code)]
+    ShowError(String),
+    #[allow(dead_code)]
+    ClearError,
+
+    // UI refresh.
+    RefreshView,
+
+    // Fallback.
+    #[allow(dead_code)]
+    NoOp,
+}