feat(brand): default favicon emblem + Powered By footer badge

Ship the Runic Gateway emblem as the baked-in default favicon so an
instance renders a tab icon with no BRAND_FAVICON set, and place a
left-justified "Powered By" badge in the site footer.

- brand.favicon now defaults to /assets/img/favicon.ico (was empty).
  BRAND_FAVICON still overrides per-instance.
- Add favicon.ico + powered-by.svg under client/public/assets/img.
- SiteFooter: badge pinned to the left of the centered content column
  (absolute on >=641px, stacked on mobile); info text stays centered.

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

client/src/components/SiteFooter.jsx

@@ -5,31 +5,37 @@ export default function SiteFooter() {
   const { contactEmail, siteTitle } = useSite()
   return (
     <footer
-      className="sans"
+      className="sans site-footer"
       style={{
         borderTop: '1px solid var(--line)',
         padding: '28px 16px',
         color: 'var(--muted)',
-        textAlign: 'center',
         fontSize: '0.9rem',
         background: 'rgba(9,13,18,0.6)',
       }}
     >
-      <div style={{ display: 'flex', flexDirection: 'column', alignItems: 'center', gap: 6 }}>
-        <span>{siteTitle} is an independent private shard project.</span>
-        <span style={{ color: 'var(--dim)', fontSize: '0.84rem' }}>
-          <a href={`mailto:${contactEmail}`} style={{ color: 'var(--accent)', textDecoration: 'none' }}>
-            {contactEmail}
-          </a>
-          &nbsp;·&nbsp;
-          <Link to="/site/status" style={{ color: 'var(--accent)', textDecoration: 'none' }}>
-            Shard Status
-          </Link>
-          &nbsp;·&nbsp;
-          <Link to="/admin/login" style={{ color: '#5d6b7d', textDecoration: 'none' }}>
-            Admin
-          </Link>
-        </span>
+      <div className="site-footer-inner">
+        <img
+          className="site-footer-badge"
+          src="/assets/img/powered-by.svg"
+          alt="Powered by Runic Gateway"
+        />
+        <div className="site-footer-info">
+          <span>{siteTitle} is an independent private shard project.</span>
+          <span style={{ color: 'var(--dim)', fontSize: '0.84rem' }}>
+            <a href={`mailto:${contactEmail}`} style={{ color: 'var(--accent)', textDecoration: 'none' }}>
+              {contactEmail}
+            </a>
+            &nbsp;·&nbsp;
+            <Link to="/site/status" style={{ color: 'var(--accent)', textDecoration: 'none' }}>
+              Shard Status
+            </Link>
+            &nbsp;·&nbsp;
+            <Link to="/admin/login" style={{ color: '#5d6b7d', textDecoration: 'none' }}>
+              Admin
+            </Link>
+          </span>
+        </div>
       </div>
     </footer>
   )