Fix Alt-based quick measurement to show document space coordinates instead of viewport space

[vineetpuranik]

Fixes #2941

This PR addresses the bug where the quick measurement feature (triggered by holding Alt while hovering over a shape) displayed distances in viewport coordinates instead of document coordinates. This caused the measured distance to vary incorrectly based on zoom level.

Changes made

• Updated the logic to compute the bounding boxes (selected_bounds, hovered_bounds) in document space using bounding_box_document.
• Applied the document_to_viewport transform only once during rendering, preserving measurement accuracy.
• Refactored variable names and added clear inline comments for maintainability.

Before

• Measured distances between shapes would change when zooming in/out.

After

• Measured distances remain consistent regardless of zoom level.
• Overlays are correctly drawn by transforming document-space bounding boxes at render time.

Visual verification

Screencast.from.08-04-2025.11.43.41.PM.webm