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/sync.rs

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/ui/sync.rs
+//
+// Synchronize UI model from DocumentManager state.
+
+use crate::application::DocumentManager;
+use crate::domain::document::core::document::Renderable;
+use crate::ui::model::AppModel;
+
+/// Synchronize AppModel from DocumentManager.
+///
+/// Updates UI state with current document info, but does NOT copy
+/// the entire document (would break Clean Architecture).
+/// Only caches render-related data for performance.
+pub fn sync_model_from_manager(model: &mut AppModel, manager: &mut DocumentManager) {
+    // Update cached render data
+    if let Some(doc) = manager.current_document_mut() {
+        // Cache image handle for rendering
+        if let Ok(render_output) = doc.render(1.0) {
+            model.current_image_handle = Some(render_output.handle);
+        } else {
+            model.current_image_handle = None;
+        }
+
+        // Cache dimensions
+        let info = doc.info();
+        model.current_dimensions = Some((info.width, info.height));
+
+        // Cache page info
+        model.current_page = Some(doc.current_page());
+        model.page_count = Some(doc.page_count());
+    } else {
+        // No document loaded - clear cached data
+        model.current_image_handle = None;
+        model.current_dimensions = None;
+        model.current_page = None;
+        model.page_count = None;
+    }
+
+    // Update navigation state
+    model.current_path = manager.current_path().map(|p| p.to_path_buf());
+    model.folder_count = manager.folder_entries().len();
+    model.current_index = manager.current_index();
+
+    // Update metadata
+    model.metadata = manager.current_metadata().cloned();
+}
+
+/// Synchronize only render data without full document info.
+///
+/// Useful when only the rendered image has changed (e.g., after transform).
+pub fn sync_render_data(model: &mut AppModel, manager: &mut DocumentManager) {
+    if let Some(doc) = manager.current_document_mut() {
+        // Re-render at current scale to get updated image handle
+        if let Ok(render_output) = doc.render(model.scale as f64) {
+            model.current_image_handle = Some(render_output.handle);
+        }
+
+        // Update dimensions (may have changed after rotation)
+        let info = doc.info();
+        model.current_dimensions = Some((info.width, info.height));
+
+        // Update page info (in case page changed)
+        model.current_page = Some(doc.current_page());
+    }
+}
+
+/// Synchronize only navigation state without render data.
+///
+/// Useful when switching documents in a folder.
+#[allow(dead_code)]
+pub fn sync_navigation(model: &mut AppModel, manager: &DocumentManager) {
+    model.current_path = manager.current_path().map(|p| p.to_path_buf());
+    model.current_index = manager.current_index();
+    model.folder_count = manager.folder_entries().len();
+}