Bump version to 4.2.1

Release highlights:
- API key authentication (X-API-Key header)
- TLS with self-signed certificates
- CLI tools: compress, rotate, convert
- jpegtran lossless JPEG rotation
- AUR packages: stegasoo-cli-git, stegasoo-api-git
- Bug fixes: DCT rotation, jpegtran -trim, CLI output format

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/templates/about.html

@@ -340,13 +340,13 @@
                 <!-- Current Version - Prominent -->
                 <div class="alert alert-success mb-4">
                     <div class="d-flex align-items-center">
-                        <span class="badge bg-success fs-6 me-3">v4.2.0</span>
+                        <span class="badge bg-success fs-6 me-3">v4.2.1</span>
                         <div>
-                            <strong>Performance optimizations:</strong>
-                            ~70% faster decode (vectorized DCT),
-                            50% less RAM (float32),
-                            async API endpoints,
-                            decode progress callbacks
+                            <strong>Security & API improvements:</strong>
+                            API key authentication,
+                            TLS with self-signed certs,
+                            CLI tools (compress, rotate, convert),
+                            jpegtran lossless JPEG rotation
                         </div>
                     </div>
                 </div>