Fix erratic mouse behavior in the swipe (slider) compare view

[jeremyfelt]

Symptom

In the swipe (slider) compare view, dragging the divider with a mouse feels erratic — the divider can jump, lag, or start following the cursor with no button held down.

Cause

Two classic before/after-slider pointer bugs in buildSwipe:

1. Native image drag hijacks the gesture. The screenshots are normal <img> elements (draggable by default), and pointerdown never called preventDefault() — so mousedown-then-move starts a native image drag (ghost image, "no-drop" cursor) instead of moving the divider.
2. The drag gets stuck on. The native drag fires pointercancel, not pointerup, and only pointerup was handled — so dragging never reset to false, and afterwards the divider tracks the cursor on plain hover.

Fix

The standard guards used by image-comparison sliders (img-comparison-slider, Juxtapose, cocoen):

• preventDefault() on pointerdown; respond to the primary button only.
• End the drag on pointerup and pointercancel / lostpointercapture, and release the pointer capture — an interrupted gesture can no longer get stuck.
• Make the images non-draggable and non-interactive: draggable="false", pointer-events: none, user-select / -webkit-user-drag: none. The stage div is now the only pointer target, so a native image drag can't start.

Test plan

• npm run lint — clean
• npm test — 111 pass
• Driven with a real mouse in Chromium: the divider tracks a drag accurately (to ~25%), stops on release (hover afterwards does not move it), a synthetic pointercancel mid-gesture leaves nothing stuck, images report draggable=false / pointer-events:none, and there are no console errors.

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