Add opt-in read-only file browser

[jlawrence6809]

Summary

• add an opt-in, read-only file browser menu
• confine navigation to explicitly configured roots
• sort folders first with deterministic natural ordering
• preserve selection across refresh/back navigation and handle empty/error states
• sanitize display text and ellipsize it to the available renderer width

The browser intentionally does not launch or mutate files. It is enabled only by adding a file_browser menu entry with configured roots; sampleconfig.json remains unchanged.

Validation

• native Release CMake build and CTest
• ASan + UBSan CTest
• nxdk XBE and XISO build
• xemu navigation, empty/error, held-input, and non-openable-file behavior
• physical revision 1.4 Xbox navigation and rendering
• independent design and implementation review: GO

Follow-up

Validated XBE launching is intentionally split into dependent follow-up #159.