SwiftUI: add Files back navigation and file info in thread file viewer

[saphid]

When a file is opened from a thread in the native SwiftUI client, the bottom drawer shows the text file without a route back to its containing directory and without file details.

Acceptance criteria:

• A Files button returns to the containing directory in the file browser.
• An info button shows useful details for the current file, including its path.
• The behavior is covered by focused tests and validated on the approved iPhone build before upstream preparation.