fix: handle charset decoding for MSG and EML

[Rasalas]

Summary

• add centralized charset/base64/quoted-printable decoding helpers
• decode MSG/EML text, MIME headers, RFC2231 filenames, text attachments, and nested EMLs through byte-safe paths
• add regression coverage for German Windows-1252 text, UTF-8 fallback behavior, quoted-printable HTML, and attachment filenames

Verification

• npm test
• npm run lint
• npm run build

Note: repo-wide npm run format:check still reports pre-existing unformatted files outside this change; touched files pass Prettier after fixing the invalid .prettierrc option.