Security hardening: remove global leaks, enforce crypto, cleanup logs
- Removed unsafe exposure of internal objects:
• deleted all references to `window.fileTransferSystem`
• unified global API setup to a single immutable `safeGlobalAPI` via Object.freeze
• removed legacy branch with `delete window[name]`
- Logging improvements:
• eliminated all direct `console.*` calls (use `_secureLog` exclusively)
• fixed string templates with proper interpolation (``${var}``)
• prevented accidental fallback to raw console in production
- Cryptography enforcement:
• updated `_validateCryptographicSecurity()` to require ECDSA and PFS
• added emergency shutdown if critical crypto features are missing
• ensured consistent feature enabling for production
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