#3591851: Allow editing stored patterns

Dave Longrequested to merge
issue/canvas-3591851:3591851-edit-stored-patterns into 1.x
  • Adds "Edit pattern" to the pattern library
  • Patterns can have components added, changed and removed
  • Patterns can be renamed
  • Save/publish works in the same way as pages and content templates

AI-Generated: Yes (Used Claude Code to develop almost all the changes and write the tests, with manual testing in between)

Testing instructions

  • Create a pattern then click "Edit pattern" in the pattern library
  • Rename the pattern in the header
  • Add, update, remove components in the pattern
  • Save the changes
  • Ensure the changes remain after reloading the editor or placing the pattern elsewhere

Closes #3591851

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