Clean warnings and harden image decoding

[5428384822a-create]

Summary

• clean MSVC warnings from gesture config, thumbnail loading, and CRT file APIs
• keep source encoding explicit with /utf-8 and suppress intentional cache-line padding warning
• harden WIC decode/info/thumbnail paths against missing files, failed HRESULTs, zero dimensions, oversized buffers, and allocation failures

Verification

• cmake --build build-vs2026 --config Release --parallel
• ctest --test-dir build-vs2026 -C Release --output-on-failure (no tests found)