Add safe deletion for SD card transaction and descriptor files

[nonoplzno]

Summary

Adds safe deletion of transaction data and wallet descriptor files directly from the Open SD card file menu.

Implementation

• Adds a dedicated Delete a file menu action.
• Lists only the same supported .psbt, .txt, and .json files already accepted by the SD host.
• Requires explicit confirmation before deletion.
• Synchronizes the filesystem after deletion.
• Reports deletion failures through HostError.
• Leaves cancellation and normal file selection unchanged.

Verification

• Python syntax validation passed.
• Behavioral test passed for:
  • confirmed deletion;
  • cancelled deletion;
  • normal file selection;
  • unsupported files remaining untouched.
• git diff --check passed.

close #359

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[nonoplzno]

@stepansnigirev @k9ert This implementation is ready for maintainer review.

Validation completed:

• Python syntax: PASS
• confirmed deletion behavior: PASS
• cancelled deletion preserves the file: PASS
• normal file selection remains unchanged: PASS
• unsupported files remain untouched: PASS
• git diff check: PASS

The deletion action is restricted to files already accepted by the SD host and requires explicit confirmation.