Canvas mode: separate viewport pan from object selection

[lidge-jun]

Source

Canvas Mode follow-up from devlog/_plan/0.24-canvas-mode/FOLLOW-UP-ISSUES.md and parent issue #22.

Goal

Separate true image viewport panning from Canvas object selection/move behavior.

Current state

• CanvasViewport includes panX and panY fields.
• Canvas zoom exists.
• The current pan tool is effectively selection mode:
  • click selects object
  • shift-click toggles selection
  • drag selected object moves it
  • drag empty area creates selection box
• There is no actual viewport pan behavior for zoomed images.

Scope

• Decide the gesture model for viewport pan vs object selection.
• Add actual viewport panning for zoomed Canvas view.
• Avoid breaking existing selection, move, and selection-box behavior.
• Define whether pan is a separate tool, a modifier gesture, or a mode inside the existing pan/select tool.
• Keep keyboard zoom and Escape close behavior intact.

Out of scope

• New annotation tools.
• SVG/PPTX export.
• Alpha export controls.
• Full layer/object inspector.

Acceptance criteria

• A zoomed canvas can be panned without moving selected objects accidentally.
• Existing object selection, multi-select, and move still work.
• Selection box remains available through an explicit, predictable gesture.
• Panning state does not affect saved normalized annotation coordinates.
• Tests cover viewport pan state, object move separation, and no regression in selection shortcuts.
• Manual QA covers mouse/trackpad behavior at 1x and zoomed states.