feat(ui-tui): define a terminal fallback for WebPreview

[roninjin10]

WebPreview is intentionally DOM-only in the isomorphic UI library because its core surface is a real sandboxed iframe. OpenTUI has no embedded browser context, so silently rendering nothing would violate target parity.

Acceptance criteria:

• define an explicit terminal interaction for web preview URLs (for example, validated URL text plus an open-in-browser action)
• preserve the shared URL and loading-state data contract where meaningful
• add OpenTUI frame and interaction tests
• remove the WebPreview parity exception only if the terminal implementation reaches honest behavioral parity

Tracked from the agents+sandbox isomorphic component lane.