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.

src/data/site.ts

@@ -26,7 +26,7 @@ export const site = {
   url: "https://www.bztmon.com",
   // Public contact (matches the CV).
   email: "jonnywright2011@live.com.au",
-  ogImage: "/og.png",
+  ogImage: "/og.png?v=2",
   locale: "en",
   // Content license for blog posts + pages. Edit here to relicense site-wide.
   license: {