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/views/canvas.rs

@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/app/view/canvas.rs
+//
+// Render the center canvas area with the current document.
+
+use cosmic::iced::widget::image::FilterMethod;
+use cosmic::iced::{ContentFit, Length};
+use cosmic::iced_widget::stack;
+use cosmic::widget::{container, text};
+use cosmic::Element;
+
+use crate::ui::components::crop::crop_overlay;
+use super::image_viewer::Viewer;
+use crate::ui::model::{ToolMode, ViewMode};
+use crate::ui::{AppMessage, AppModel};
+use crate::application::DocumentManager;
+use crate::config::AppConfig;
+use crate::fl;
+
+/// Render the center canvas area with the current document.
+pub fn view<'a>(
+    model: &'a AppModel,
+    _manager: &'a DocumentManager,
+    config: &'a AppConfig,
+) -> Element<'a, AppMessage> {
+    if let Some(handle) = &model.current_image_handle {
+        let content_fit = match model.view_mode {
+            ViewMode::Fit => ContentFit::Contain,
+            ViewMode::ActualSize | ViewMode::Custom => ContentFit::None,
+        };
+
+        let img_viewer = Viewer::new(handle)
+            .with_state(model.scale, model.pan_x, model.pan_y)
+            .on_state_change(|scale, offset_x, offset_y, canvas_size, image_size| {
+                AppMessage::ViewerStateChanged {
+                    scale,
+                    offset_x,
+                    offset_y,
+                    canvas_size,
+                    image_size,
+                }
+            })
+            .width(Length::Fill)
+            .height(Length::Fill)
+            .content_fit(content_fit)
+            .filter_method(FilterMethod::Nearest)
+            .min_scale(config.min_scale)
+            .max_scale(config.max_scale)
+            .scale_step(config.scale_step - 1.0)
+            .disable_pan(model.tool_mode == ToolMode::Crop);
+
+        if model.tool_mode == ToolMode::Crop {
+            let overlay = crop_overlay(&model.crop_selection, config.crop_show_grid);
+
+            stack![img_viewer, overlay].into()
+        } else {
+            container(img_viewer)
+                .width(Length::Fill)
+                .height(Length::Fill)
+                .into()
+        }
+    } else {
+        container(text(fl!("no-document")))
+            .width(Length::Fill)
+            .height(Length::Fill)
+            .center(Length::Fill)
+            .into()
+    }
+}