Add file upload pickers and permission prompts

Summary

Add native file upload selection and clear permission prompts for supported page capabilities.

Scope

• Present a native file picker for page file inputs and return only the user-selected files through an explicit Engine API.
• Show the requesting site and requested capability in prompts for permissions Photon Engine exposes.
• Support allow, deny, and cancel outcomes; never grant access silently.
• Keep grants profile-scoped and make temporary versus remembered grants visible.
• Handle cancellation, navigation, tab closure, and window teardown without stale callbacks.

Acceptance criteria

• Page content cannot choose arbitrary local paths or bypass the native picker.
• Permission prompts are tied to the requesting origin and current document.
• Cancelling a picker or prompt leaves no unintended grant or file access.
• Native dialogs do not block the GPUI event loop.