Implement export and import functionality for saved clips and boards

[kurdin]

Implement the ability to export and import selected clips content:

Export Features:

• Allow selection of multiple saved clips or entire boards
• Export selected items in a standard format (JSON)
• Include metadata (creation date, tags, categories)
• Provide clear success/error feedback during export

Import Features:

• Support importing previously exported clipboard data

• Validate imported data structure and format

• Handle duplicate detection

• Provide progress indication for large imports

• User Interface:

• Add export/import options to the main menu

• Show file picker for selecting export location

• Display confirmation dialogs for overwrite scenarios

• Show success/error notifications