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/domain/document/core/page.rs

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// src/domain/document/core/page.rs
+//
+// Page abstraction for multi-page documents.
+
+use cosmic::widget::image::Handle as ImageHandle;
+
+/// Represents a single page in a multi-page document.
+#[derive(Debug, Clone)]
+pub struct Page {
+    /// Page index (0-based).
+    pub index: usize,
+    /// Page width in pixels.
+    pub width: u32,
+    /// Page height in pixels.
+    pub height: u32,
+    /// Optional thumbnail handle.
+    pub thumbnail: Option<ImageHandle>,
+}
+
+impl Page {
+    /// Create a new page.
+    #[must_use]
+    pub fn new(index: usize, width: u32, height: u32) -> Self {
+        Self {
+            index,
+            width,
+            height,
+            thumbnail: None,
+        }
+    }
+
+    /// Create a page with a thumbnail.
+    #[must_use]
+    pub fn with_thumbnail(index: usize, width: u32, height: u32, thumbnail: ImageHandle) -> Self {
+        Self {
+            index,
+            width,
+            height,
+            thumbnail: Some(thumbnail),
+        }
+    }
+
+    /// Set the thumbnail for this page.
+    pub fn set_thumbnail(&mut self, thumbnail: ImageHandle) {
+        self.thumbnail = Some(thumbnail);
+    }
+
+    /// Check if this page has a thumbnail.
+    #[must_use]
+    pub fn has_thumbnail(&self) -> bool {
+        self.thumbnail.is_some()
+    }
+
+    /// Get the aspect ratio of the page.
+    #[must_use]
+    pub fn aspect_ratio(&self) -> f32 {
+        if self.height == 0 {
+            1.0
+        } else {
+            #[allow(clippy::cast_precision_loss)]
+            {
+                self.width as f32 / self.height as f32
+            }
+        }
+    }
+
+    /// Get page dimensions as a tuple.
+    #[must_use]
+    pub fn dimensions(&self) -> (u32, u32) {
+        (self.width, self.height)
+    }
+}