Fix "Implement in a new thread" menu copy

[UtkarshUsername]

What Changed

Changed the plan action menu item text from Implement in new thread to Implement in a new thread.

Also updated the nearby code comment so it matches the user-facing label.

Why

The existing label reads awkwardly in the UI. This is a small copy fix that improves clarity and polish without changing behavior.

Closes #1062

UI Changes

Before	After

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes

Note

Fix grammar in 'Implement in a new thread' menu item label

Adds the missing article 'a' to the menu item label in ChatView.tsx. No logic or behavior is changed.

^{Macroscope summarized 54bba2e.}

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