feat(brand): emblem behind hero text + Powered By footer with logo

Address review: the SVG footer badge rendered too small, and the default
hero should feature the Runic Gateway emblem rather than the moon.

- Footer: drop powered-by.svg; render the emblem PNG beside a "Powered by
  Runic Gateway" label (left-justified, info text stays centered).
- Default hero: brand.hero (and the client fallbacks) now default to the
  emblem PNG. The untouched default hero centers the square emblem behind
  the text as a medallion with a symmetric legibility overlay (per-layer
  background-size so the overlay stays full-bleed). BRAND_HERO still
  overrides.
- Admin login / player shell / maintenance backgrounds center the emblem
  as a capped medallion instead of a cropped full-bleed cover.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XmHdsbnLzDMAVQkAoTQSBe

client/src/components/SiteFooter.jsx

@@ -15,11 +15,14 @@ export default function SiteFooter() {
       }}
     >
       <div className="site-footer-inner">
-        <img
-          className="site-footer-badge"
-          src="/assets/img/powered-by.svg"
-          alt="Powered by Runic Gateway"
-        />
+        <div className="site-footer-badge">
+          <img src="/assets/img/runic-emblem.png" alt="" aria-hidden="true" />
+          <span>
+            Powered by
+            <br />
+            <strong>Runic Gateway</strong>
+          </span>
+        </div>
         <div className="site-footer-info">
           <span>{siteTitle} is an independent private shard project.</span>
           <span style={{ color: 'var(--dim)', fontSize: '0.84rem' }}>