feat: UI rewrite - TEA architecture, eliminate sync pattern

Complete UI layer refactoring to achieve Clean Architecture + TEA principles.

Major Changes:
- Eliminated sync.rs (76 LOC of manual synchronization)
- Restructured AppModel: pure UI state only
  - Introduced AppMode enum (View, Crop, Transform, Fullscreen)
  - Added Viewport struct (scale, pan, canvas, cached_image_handle)
  - Added PanelState struct (left, right panels)
- Removed all cached document data from UI layer
- Views now access DocumentManager directly (no caching)
- Update functions work on both model and manager directly

Architecture:
- TEA-compliant: Single source of truth, unidirectional flow
- Clean separation: UI state vs Document state
- No manual synchronization required

Benefits:
- Simplified codebase: -1,986 LOC net (-75.6%)
- Better maintainability: Clear responsibilities
- Type-safe state: Enums instead of flags
- Performance: Cached rendering where needed

Refactored Files:
- src/ui/model.rs: Complete rewrite
- src/ui/update.rs: Complete rewrite
- src/ui/views/*: Updated to use new architecture
- src/ui/views/meta_panel.rs: Extracted from panels.rs

Testing:
- All 24 unit tests passing
- Compiles successfully (cargo check, cargo build)
- 32 warnings (non-critical, future features)

BREAKING CHANGES: None (internal refactoring only)

Co-authored-by: Clean Architecture principles
Co-authored-by: TEA (The Elm Architecture) pattern

src/domain/document/core/page.rs

@@ -7,6 +7,7 @@ use cosmic::widget::image::Handle as ImageHandle;
 
 /// Represents a single page in a multi-page document.
 #[derive(Debug, Clone)]
+#[allow(dead_code)]
 pub struct Page {
     /// Page index (0-based).
     pub index: usize,