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/message.rs

@@ -37,6 +37,8 @@ pub enum AppMessage {
     ZoomReset,
     /// Fit document to window.
     ZoomFit,
+    /// Update zoom and pan from viewer (mouse interaction).
+    ViewerStateChanged { scale: f32, offset_x: f32, offset_y: f32 },
 
     // === Pan ===
     /// Pan image left.