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

pom.xml

@@ -4,7 +4,7 @@
          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
 
-    <groupId>io.github.sneakybugmaster.mailsender</groupId>
+    <groupId>pro.thinhha.mailsender</groupId>
     <artifactId>mailsender-parent</artifactId>
     <version>0.1.0-SNAPSHOT</version>
     <packaging>pom</packaging>
@@ -40,17 +40,17 @@
                 <scope>import</scope>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-core</artifactId>
                 <version>${project.version}</version>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-spring-boot-autoconfigure</artifactId>
                 <version>${project.version}</version>
             </dependency>
             <dependency>
-                <groupId>io.github.sneakybugmaster.mailsender</groupId>
+                <groupId>pro.thinhha.mailsender</groupId>
                 <artifactId>mailsender-pgp-spring-boot-starter</artifactId>
                 <version>${project.version}</version>
             </dependency>