fix(viewports): fit extents per pane

Closes #554

src/scene/view/camera.rs

@@ -464,6 +464,10 @@ impl Camera {
         self.depth_half_range = self.depth_extent_in_view(min, max);
     }
 
+    pub(crate) fn fitted_model_bounds(&self) -> Option<(Vec3, Vec3)> {
+        self.model_bounds
+    }
+
     // ── ViewCube snap ─────────────────────────────────────────────────────
 
     /// Snap to a canonical view direction (called by ViewCubeSnap).

src/scene/view/render.rs

@@ -1923,6 +1923,10 @@ impl Scene {
         };
         let active = self.active_model_tile.get();
         let is_active = tile_idx == active;
+        if is_active && canvas.1 > 0.0 {
+            self.set_render_aspect(canvas.0 / canvas.1);
+            self.set_render_pixel_scale(canvas.0, canvas.1);
+        }
         let nav_build_started = iced::time::Instant::now();
         let perf_nav = if is_active {
             self.take_nav_perf()