fix: curved connector's control-point drag now tracks the cursor at any zoom/pan

[bvnaik05]

Problem

Dragging the midpoint control handle on a curved connector (the dot used to bend the curve) would visibly jump away from the cursor mid-drag, worse the further the drag traveled — described as the dot "randomly spinning off somewhere" instead of tracing the pointer.

Root cause

ConnectorView.vue's toLogical() converted the screen point to logical (world) coordinates via:

const ctm = node.ownerSVGElement.getScreenCTM()

ownerSVGElement is the root canvas <svg>, which has no transform of its own — the pan/zoom lives on a descendant <g :transform="groupTransform"> that every connector and shape renders inside. Calling getScreenCTM() on the root therefore returns a CTM that entirely excludes the pan/zoom transform. At the default 100% zoom with no pan this is indistinguishable from correct (identity difference), which is why it read as "no bug found" under static review — it only shows up once the canvas is panned or zoomed, which is exactly how anyone actually uses a connector mid-session.

Four sibling files already get this right: HoverArrows, MindmapHoverHandles, FlowchartHoverHandles, and HoverOutline all call getScreenCTM() on an element that sits inside the transformed <g> (their own root, via a template ref), which correctly includes the pan/zoom. ConnectorView was the outlier.

Fix

Call getScreenCTM() on the dragged node itself (the handle circle, or the connector's hit-path), not on ownerSVGElement. The node already sits inside the transformed <g>, so its own CTM is exactly the coordinate space its own cx/cy (or the connector's endpoint coordinates) are expressed in.

This helper is shared by all three connector drag gestures (control-point, endpoint re-attach, and whole-body move), so the fix corrects all three — endpoint dragging and body dragging had the identical latent bug, just less noticeable since endpoints snap to shapes.

Testing

• yarn vitest run connector — 11 files / 63 tests pass.
• Verified live: created a curved connector, zoomed to 118% and panned the canvas, selected the connector, dragged the control handle to a specific screen point (400, 300). Measured the handle's actual rendered position after the drag: (400.32, 300.46) — sub-pixel accurate. Before the fix this same drag would land far from the release point, growing worse with distance from the pan/zoom origin.

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