Fix og:image to use existing banner.png asset

logo2.png does not exist in docs/res/, causing broken social previews.
Use banner.png which is a known-good asset.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

docs/index.html

@@ -19,7 +19,7 @@
     <meta name="keywords" content="exascale, fluid dynamics, cfd, computational fluid dynamics, compressible, hpc, bryngelson, colonius, subgrid, multiphase, frontier, summit, el capitan, aurora, amd gpu, gpu, nvidia, gordon bell"/>
     <meta property="og:title" content="MFC - Exascale Multiphase Flow Solver" />
     <meta property="og:description" content="Open-source exascale multiphase flow solver. 2025 Gordon Bell Prize Finalist. Scales to 200+ trillion grid points on 43,000+ GPUs." />
-    <meta property="og:image" content="https://mflowcode.github.io/res/logo2.png" />
+    <meta property="og:image" content="https://mflowcode.github.io/res/banner.png" />
     <meta property="og:url" content="https://mflowcode.github.io" />
     <meta property="og:type" content="website" />
     <meta name="twitter:card" content="summary_large_image" />