Validate legacy settings before flash writes

[Jacksper13]

Summary

• validate serialized legacy settings size before selecting or erasing a flash slot
• replace the invalid lowercase assert false with an explicit ValueError
• add a regression test covering oversized settings and the no-flash-selection invariant

Background

The legacy internal-flash settings backend attempted to reject JSON larger than its 8,160-byte data area with assert false. Because Python boolean literals are capitalized, that path raised NameError. It also selected a flash slot before checking the serialized size, allowing an oversized save attempt to perform flash management before failing.

This backend is retained for the Passport Founders Edition v1-to-v2 settings migration; current user settings use the external settings backend.

Validation

• focused overflow-path test with MicroPython modules stubbed
• Python syntax compilation
• pycodestyle on the changed Python files
• git diff --check