fix: repair undo/redo malfunction (#1321)

Three issues fixed:

1. editorReplace() incorrectly dispatched 'undo' instead of 'replace'
   due to a copy-paste error, causing the Replace menu item to trigger
   an undo action.

2. History debounce timer reduced from 2000ms to 500ms so that undo
   steps are more granular — previously a single Ctrl+Z could wipe
   an entire paragraph of recently typed text.

3. redo() now commits pending state before stepping forward (mirroring
   undo behaviour) instead of silently discarding it, which caused
   redo to appear broken.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>