Implement persistent sidebar state management

[fx]

Description

Add functionality to remember the sidebar's pinned/unpinned state across sessions and page reloads.

Acceptance Criteria

• Store sidebar state in local storage or dashboard configuration
• Restore sidebar state on page load
• Update state when user pins/unpins sidebar
• Sync state across dashboard configuration export/import

Technical Notes

• Consider using TanStack Store for state management
• Ensure state persists in YAML configuration

Epic: #72