feat: add toolbar checkboxes example

[mattConnHarbour]

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[linear]

SD-2164 Checkbox support - toolbar checkbox button demo

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clear delayed toggle hook before replacing editor instance

The onReady callback defers setupClickToToggle with setTimeout, but the watch(file, ...) path destroys/recreates superdoc without cancelling any previously scheduled timeout. If a user changes documents quickly, the old timeout can run after superdoc points to the new instance and attach a duplicate selectionUpdate listener there, causing a single click/selection entry to toggle a checkbox twice (effectively no-op). Store the timeout id and clear it during teardown (or bind setup to a captured editor instance) to avoid duplicate handlers.

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