improving 'open in X' button UI

[edoedac0]

Summary

• Adds a unified, square button group for “Open in” actions to match the header styling.
• Uses a full‑height divider to visually separate the primary action from the dropdown.
• Keeps the interaction the same, but makes the control feel like a single, intentional component.

Example (Before and After)

Before:

After (transparent icons are in #12641):

[github-actions]

Hey! Your PR title improving 'open in X' button UI 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]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found