Add front-button mapping diagnostic logging

[onehoon]

Summary

• Log the frozen-default fallback when FrontButtonMapping is missing or not a JSON object.
• Log all four effective front-button bindings after a valid settings load.
• Log all four bindings after a successful front-button mapping save.
• Add focused coverage for fallback, load, and save diagnostics.

Scope

Diagnostic-only change for WING / OEM1 mapping incidents. Existing defaults, validation, dispatch, persistence behavior, lifecycle, and mapping architecture are unchanged. No migration from legacy Oem1Mapping / WingMapping fields.

Validation

• Front-button mapping/settings and dispatch focused tests: 24/24 passed.
• Full test suite: 2737/2737 passed.
• Release build: 0 warnings, 0 errors.
• git diff --check passed.
• Hardware validation is not applicable; this PR adds logging only.