Avoid double editor transaction application

[ComputelessComputer]

Move editor change notifications into a ProseMirror plugin view update and cancel pending debounced note saves before external state sync.

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Note

Medium Risk
Touches how note/chat content is persisted and synced; behavior is covered by new plugin tests but regressions in save timing or missed updates are possible.

Overview
Stops using a custom dispatchTransaction on chat and note ProseMirror editors (and the dispatchEditorTransaction helper for onUpdate / handleChange), so transactions are not applied twice when the host library already dispatches them.

Change detection moves into a new docChangeListenerPlugin that runs in the plugin view update hook and calls back only when the document changed as part of a transaction—not on selection-only updates or when the doc is replaced via view.updateState during controlled initialContent sync.

For the note editor, pending debounced saves are cancelled (onUpdate.cancel()) before that external sync, and the debounced handler receives JSON from the plugin instead of re-reading the view inside flushChange.

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