Implement hybrid print preview with CSS-based approach

- Add hidden print-preview-copy div at app level for clean print isolation
- Use @media print rules to show only print copy (960x720) in landscape
- Auto-sync print copy with preview content on modal render via use_effect
- Copy CSS variables and data attributes for proper hour filtering
- Set explicit dimensions matching print-preview-paper content area
- Force landscape orientation with @page rule for better calendar printing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/app.rs

@@ -1702,6 +1702,9 @@ pub fn App() -> Html {
                     on_close={on_mobile_warning_close}
                 />
             </div>
+            
+            // Hidden print copy that gets shown only during printing
+            <div id="print-preview-copy" class="print-preview-paper" style="display: none;"></div>
         </BrowserRouter>
     }
 }