Add certificate validation and PGP/MIME encryption; rename to pro.thinhha.mailsender

- validation: CertificateValidator checks revocation, expiry, matching
  user ID, a valid communications encryption subkey and the algorithm
  policy; uses the earlier expiry when PGPainless and Bouncy Castle disagree
- crypto: PgpEncryptor (ASCII armor, hidden recipients for Bcc)
- mime: PgpMimeEncryptor builds RFC 3156 multipart/encrypted from the
  whole CRLF-canonical inner entity, keeping envelope headers in the clear
- autoconfigure: validator and encryptor beans
- demo: encrypts when a valid key is found, plaintext otherwise
- rename groupId and Java packages to pro.thinhha.mailsender

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

mailsender-demo/src/main/resources/application.yml

@@ -20,4 +20,4 @@ demo:
 
 logging:
   level:
-    io.github.sneakybugmaster.mailsender: DEBUG
+    pro.thinhha.mailsender: DEBUG