docs: strip white background from Sentinel shield PNG

The PNG had opaque white pixels inside the rounded-square frame, so
any wrapper background was hidden underneath it. Replaced near-white
pixels with transparency so the shield now blends straight into the
matrix page background.

Author: Sentinel-Bluebuilder

docs/assets/sentinel-shield.png

@@ -155,14 +155,12 @@
 }
 
 nav .logo .brand-mark {
-  width: 28px;
-  height: 28px;
+  width: 24px;
+  height: 24px;
   display: inline-flex;
   align-items: center;
   justify-content: center;
-  background: #000;
-  border-radius: 4px;
-  padding: 2px;
+  filter: drop-shadow(0 0 6px rgba(30, 136, 229, 0.45));
 }
 
 nav .logo .brand-mark img { width: 100%; height: 100%; object-fit: contain; display: block; }
@@ -1105,9 +1103,8 @@
 <section class="hero">
   <div class="container">
     <div class="protocol-badge fade-in">
-      <span style="display:inline-flex;align-items:center;justify-content:center;width:20px;height:20px;background:#000;border-radius:4px;padding:1px;">
-        <img src="assets/sentinel-shield.png" alt="" style="width:100%;height:100%;object-fit:contain;display:block;">
-      </span>
+      <img src="assets/sentinel-shield.png" alt="" style="width:16px;height:16px;object-fit:contain;display:inline-block;vertical-align:middle;filter:drop-shadow(0 0 4px rgba(30,136,229,0.5));">
+
       Live on Base Mainnet &middot; Sentinel dVPN
     </div>
     <h1 class="fade-in">Private internet<br>for <span class="highlight">AI agents</span><span class="caret"></span></h1>