Allow setting the built-in browser as default for agent-opened URLs (currently they jump to the system default — Chrome)

[BlessedChild]

Clicking an .html file in the workspace opens it in Finch's built-in browser — and the experience is great (no context switch, no broken focus). But when the agent opens a website in a conversation (e.g. previewing a site it just generated), Finch hands the URL off to the system default browser (Chrome). It would be valuable to expose a setting so that agent-triggered URLs can default to the built-in browser too.

Steps to reproduce

1. In the workspace, click on any .html file → opens in built-in browser ✓
2. Ask the agent to open the same site in a conversation → opens in the system default browser (Chrome)

Expected behavior

A setting in Preferences, for example:

Open agent-triggered URLs in: ◉ Built-in browser / ○ System default browser

so the user can choose where agent-opened URLs land.

Actual behavior

Agent-triggered URLs always go to the system default browser, with no way to redirect them to the built-in browser.

Screenshots

• 07-builtin-browser-good.png — example of the built-in browser experience when the user opens an .html directly (used as the "good experience" reference that should also be available for agent-opened links)

Environment

• App: Finch 1.1.0 (build 174) — Desktop
• OS: macOS 26.4.1 (25E253)
• System default browser: Chrome