ui: retheme favicon + OG card to the cyan→magenta theme

The tab logo and social preview were still on the old teal palette. Retone both
to the electric cyan→magenta theme matching the hero nameplate:
- favicon.svg: jw monogram + prompt-line + cursor on a cyan→magenta gradient.
- og.png (gen-og.mjs): name in the cyan→violet→magenta hero gradient, cyan
  eyebrow/separators, magenta URL, cyan→magenta top bar + dual corner glows.
- bump ?v=2 on both refs (public/ assets are CF-edge-cached + social platforms
  cache OG hard) so the new art shows immediately.

public/favicon.svg

@@ -1,11 +1,17 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="JW">
+  <defs>
+    <linearGradient id="jw" x1="0" y1="0" x2="64" y2="64" gradientUnits="userSpaceOnUse">
+      <stop offset="0" stop-color="#3fbaf5"/>
+      <stop offset="1" stop-color="#e879f9"/>
+    </linearGradient>
+  </defs>
   <rect width="64" height="64" rx="14" fill="#090c14"/>
-  <rect x="0.5" y="0.5" width="63" height="63" rx="13.5" fill="none" stroke="#2dd4bf" stroke-opacity="0.5"/>
+  <rect x="0.5" y="0.5" width="63" height="63" rx="13.5" fill="none" stroke="url(#jw)" stroke-opacity="0.6"/>
   <text x="32" y="33" font-family="ui-monospace, Menlo, Consolas, monospace" font-size="22"
-        font-weight="700" fill="#5eead4" text-anchor="middle" dominant-baseline="central"
+        font-weight="700" fill="url(#jw)" text-anchor="middle" dominant-baseline="central"
         letter-spacing="1">jw</text>
-  <rect x="18" y="46" width="22" height="3" rx="1.5" fill="#2dd4bf"/>
-  <rect x="43" y="46" width="6" height="3" rx="1.5" fill="#2dd4bf">
+  <rect x="18" y="46" width="22" height="3" rx="1.5" fill="url(#jw)"/>
+  <rect x="43" y="46" width="6" height="3" rx="1.5" fill="#e879f9">
     <animate attributeName="opacity" values="1;0;1" dur="1.1s" repeatCount="indefinite"/>
   </rect>
 </svg>