REFACTOR: swap current editor autosave logic with the CKEditor Autosave plugin

Refactoring Task

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Problem description

Editor autosaves can be made simpler and more reliable with the CKEditor Autosave plugin.

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Affected Components/Files:

• src/app/_components/exhibition-content-editor.tsx
• files, which import the mentioned file

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Tasks:

• Remove current autosave logic (onBlur function)
• Implement autosave with the CKEditor Autosave plugin (already imported in the config)

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Acceptance Criteria:

• Current autosave logic is removed
• Autosave plugin is implemented