fix: optimize several issues and experiences

[ACTCD]

Fixed issues found in QA from #844:

• If the sidebar is hidden at "No Item Selected", the switch cannot be displayed.
  • Entering the extension page from Safari settings (by reduce competition conditions)
  • If items are empty (by prevent hide sidebar)
• Can resize sidebar in the settings modal opened (by reduce z-index)

[Copilot]

Pull Request Overview

This PR optimizes several issues found in QA testing, focusing on user experience improvements and bug fixes. The changes address item activation logic, UI interaction validation, and z-index layer management.

Key changes:

• Improved item activation validation by removing state dependency and adding null checks
• Added validation to prevent hiding sidebar when no item is selected
• Adjusted z-index values for UI layering and removed dropdown positioning attribute

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Sidebar.svelte	Updated item activation logic and dropdown positioning
App.svelte	Added sidebar toggle validation and adjusted z-index values