ScrollArea: fast thumb drag can leave webkitUserSelect and pointer capture stuck

[preening]

Description

When dragging the ScrollArea thumb quickly, pointerup sometimes fails to fire cleanly, leaving the page in a broken state:

1. Text becomes unselectable — document.body.style.webkitUserSelect stays "none"
2. Scrollbar sticks to mouse — pointer capture is never released, so the thumb follows the cursor without clicking

Reproduction

1. Render a ScrollArea with enough content to scroll
2. Grab the scrollbar thumb and drag it very quickly
3. Release the mouse button
4. Try to select text anywhere on the page — cursor won't change and text is unselectable
5. Move mouse into scrollbar region — thumb teleports to mouse and sticks

Root Cause

In scroll-area.svelte.ts lines 770-795:

onpointerdown(e: BitsPointerEvent) {
    target.setPointerCapture(e.pointerId);
    this.root.domContext.getDocument().body.style.webkitUserSelect = "none";
    // ...
}

onpointerup(e: BitsPointerEvent) {
    target.releasePointerCapture(e.pointerId);
    this.root.domContext.getDocument().body.style.webkitUserSelect = this.prevWebkitUserSelect;
    // ...
}

If pointerup is missed (fast drag, system interrupt, etc.), there's no fallback to restore state.

Suggested Fix

• Add a timeout fallback to reset state if no pointerup within ~100ms of last pointermove
• Or use the existing TextSelectionLayer utility pattern for scoped user-select management
• Or add a pointercancel handler as a safety net

Environment

• bits-ui: latest
• Browser: Chrome/Safari (both affected)
• OS: macOS