feat: add pan and zoom toggle controls with full camera state save/re…

[patrickoleary]

Adds pan, zoom, and aspect ratio controls to the Viewport Layout toolbar as collapsible toggle groups, with full camera state persistence.

UI Changes

• Aspect ratio toggle — reveals a slider (0–4, step 0.25) with tick marks
• Zoom toggle — reveals zoom in/out buttons
• Pan toggle — reveals up/down/left/right pan buttons
• Reset view button resets camera to default
• Toggles are mutually exclusive — activating one collapses the others
• Active groups highlighted with a subtle background
• Toolbar streamlined: removed "Viewport layout" label, grouped/uniform is now icon-only with tooltip, Size is icon-only with tooltip, hotkey badge removed
• Keyboard shortcut z for reset and g for grouped still work

Everything collapses. I think the dimension sliders would look a lot better this way too.

Camera State Save/Restore
State export now saves the full VTK camera configuration:

• zoom (ParallelScale)
• position (camera position)
• focal_point (camera focal point)
• view_up (camera up direction)
• clipping_range (near/far clipping planes)

API Simplification

• Replaced zoom_in/zoom_out with zoom(factor)
• Replaced pan_up/pan_down/pan_left/pan_right with pan(dx, dy)
• Button-specific logic (direction, factor) lives in toolbar lambdas

closes #80