fix(slides): prevent sticky move after fast click

[hamedrabah]

What & why

Fixes #260.

A fast single click can release while waitToChangeTarget() is still pending. The code can then pass the stale mouse-down event to Moveable after mouse-up. Moveable starts a drag without seeing the matching release, so the selected object follows the cursor.

This change installs a capture-phase mouse-up guard before the async wait. It cancels the stale handoff when release wins the race. A normal held-button drag still uses the existing path.

How I verified it

• Reproduced the unpatched failure with a real browser mouse. The object moved from (850,170) to (1014,251) after mouse-up.
• Ran 30 fast-click trials against the patch. All 30 kept the object at (850,170).
• Verified that a deliberate held-button drag still moved the object to (960,223).
• Ran node_modules/.bin/tsc -b successfully.
• Ran npm run build:single successfully.
• Ran node ../scripts/shell-gate.mjs dist-single/Bento_Slides.bento.html successfully.

Checklist

• Read the relevant parts of CLAUDE.md / docs before changing them
• npm run build:single succeeds (from slides/)
• Ran node scripts/test-sync.ts if I touched slides/src/sync/ (not applicable)
• New UI strings added to every catalog in slides/src/i18n/ (no new strings)
• Document format changes are additive and backward-compatible (no format change)
• Did not bump the version or cut a release (maintainers sign releases)

[hamedrabah]

@nyblnet, could you review this focused fix for #260 when convenient? The change is limited to preventing drag activation after a completed click, with browser regression verification documented in the PR.