feat(viewer): implement keyboard and button zoom/pan controls

- Fork cosmic::iced viewer widget to enable external state control
- Add bidirectional state synchronization between viewer and AppModel
- Implement ViewerStateChanged message for mouse interaction feedback
- Fix pixel-perfect rendering at 100% zoom (ActualSize mode)
- Ensure smooth interaction between mouse, keyboard, and button controls

This allows zoom/pan to work via:
- Keyboard shortcuts (+/-, Ctrl+arrows)
- Future toolbar buttons
- Mouse wheel and drag (existing functionality preserved)

The viewer state now properly syncs in both directions:
- External controls (keyboard/buttons) → update viewer state via diff()
- Mouse interactions → update AppModel via ViewerStateChanged message

Closes: Zoom/pan via keyboard and buttons now functional

src/app/update.rs

@@ -37,6 +37,12 @@ pub fn update(model: &mut AppModel, msg: AppMessage) {
             model.view_mode = ViewMode::Fit;
             model.reset_pan();
         }
+        AppMessage::ViewerStateChanged { scale, offset_x, offset_y } => {
+            // Update model state from viewer (mouse interaction)
+            model.view_mode = ViewMode::Custom(scale);
+            model.pan_x = offset_x;
+            model.pan_y = offset_y;
+        }
 
         // ===== Pan control (Ctrl + arrow keys) ===========================================
         AppMessage::PanLeft => {