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/app/message.rs

@@ -1,90 +0,0 @@
-// 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::app::ContextPage;
-use crate::app::view::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,
-    },
-
-    // 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,
-    },
-    CropDragEnd,
-
-    // Panels.
-    ToggleContextPage(ContextPage),
-    ToggleNavBar,
-
-    // 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,
-}