feat(desktop): add radio group button for switching agent mode

[PallieterVerhoeven]

Issue for this PR

Closes #38277

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR adds a toggle button to switch between the plan/build modes. It only shows as toggle button when there are exactly 2 options, in all other cases it will fallback to the dropdown behaviour.
The new button reduces 1 click and is just easier to use when there only to options.
The scope is only the desktop app.

How did you verify your code works?

Build the electron application locally and tested the changed views in the old v1 and new v2 ui.

Screenshots / recordings

Before:

After:
V1 ui:

V2 ui:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

PS. The new UI currently has a bug causing it to hide the plan/build modes by default, this should be fixed in a separate PR.

[github-actions]

Hey! Your PR title Add toggle button for when there are 2 options doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍