docs(slides): improve slides images and theme

docs/slides/assets/websocket-pros-cons.svg

@@ -17,8 +17,7 @@
   <rect width="960" height="540" fill="#262a3a"/>
   <path d="M38 412 L244 320 L330 484 L96 548 Z" fill="#43d9a3" opacity=".12"/>
   <path d="M720 34 L936 112 L828 236 L640 136 Z" fill="#fb7185" opacity=".12"/>
-  <text x="72" y="76" fill="#f8fafc" font-size="34" font-weight="800">WebSocket Trade-offs</text>
-  <text x="72" y="112" fill="#a8adbd" font-size="17">Strengths and limitations at a glance</text>
+  <g id="diagram-content-shift" transform="translate(0,-28)">
   <g filter="url(#shadow)">
     <rect x="92" y="154" width="350" height="286" rx="28" fill="#ffffff" opacity=".08" stroke="#ffffff"/>
     <rect x="518" y="154" width="350" height="286" rx="28" fill="#ffffff" opacity=".08" stroke="#ffffff"/>
@@ -41,4 +40,5 @@
   <circle cx="562" cy="298" r="7" fill="#ef4444"/>
   <circle cx="562" cy="346" r="7" fill="#ef4444"/>
   <circle cx="562" cy="394" r="7" fill="#ef4444"/>
+  </g>
 </svg>