Reject JWK keys with conflicting use and key_ops

[Syed-Moiz-Ali]

Fixes #58

Validates JWK \use/\key_ops\ consistency during key import across both FFI and JS backends, matching Chromium's \VerifyUsages\ behavior.

FFI backend changes:

• 
  sa_common.dart, \ec_common.dart, \�es_common.dart, \hmac.dart: validate \key_ops\ entries are consistent with the algorithm's expected \use\ value
  • \sig\ algorithms allow only [sign, verify]\
  • \enc\ algorithms allow only [encrypt, decrypt, wrapKey, unwrapKey, deriveKey, deriveBits]\
• Removes TODO comments about missing \key_ops\ validation

JS backend changes:

• Adds _checkJwkUseAndKeyOps()\ that cross-validates \use\ vs \key_ops\ when both are present
• Now strips \use\ (alongside existing \key_ops\ and \ext) before forwarding to \crypto.subtle.importKey\
• Removes the ECDH Chrome workaround (\crbug.com/641499) since \use\ is now always stripped for all algorithms
• Removes TODO about missing \use\ validation

Tests:

• \ est/jwk_key_ops_validation_test.dart: verifies rejection of conflicting \use/\key_ops\ across HMAC, AES, ECDSA, and RSA imports