Clean up tab logic in App component

[cte]

Description

I don't think we need a separate state variable for each tab. I pulled this out of the deep-research PR.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Checklist:

• My code follows the patterns of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

Additional context

Related Issues

Reviewers

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Important

Refactor App component tab logic to use a single state variable and add tests for tab switching.

• Refactor Tab Logic:
  • Replace multiple state variables with a single tab state in App.tsx.
  • Use tabsByMessageAction mapping to determine active tab based on ExtensionMessage actions.
• Testing:
  • Add App.test.tsx to test tab switching logic using @testing-library/react.
  • Mock components like ChatView, SettingsView, HistoryView, McpView, and PromptsView for testing.
  • Verify default view is chat and test switching to other views and back to chat.

^{This description was created by for 911eb92. It will automatically update as commits are pushed.}

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