Cap RSA at 3072 bits, add zstd compression for QR codes

- RSA key size capped at 3072 bits (4096 too large for QR codes)
- Added zstd compression for QR code RSA keys (better ratio than zlib)
- New prefix STEGASOO-ZS: for zstd, backward compatible with STEGASOO-Z: (zlib)
- Added zstandard dependency to web/api/compression extras
- Updated all docs, CLI options, and web UI to reflect 3072 max
- Version bump to 4.2.0

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

frontends/web/templates/about.html

@@ -573,7 +573,7 @@
                                         </tr>
                                         <tr>
                                             <td><i class="bi bi-shield me-2"></i>RSA keys</td>
-                                            <td><strong>2048, 3072, 4096 bit</strong></td>
+                                            <td><strong>2048, 3072 bit</strong></td>
                                         </tr>
                                         <tr>
                                             <td><i class="bi bi-chat-quote me-2"></i>Passphrase</td>