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/routes/admin/AdminLogin.jsx

@@ -151,7 +151,8 @@ export default function AdminLogin() {
         backgroundColor: 'var(--bg-deep)',
         backgroundImage: BG,
         backgroundPosition: 'center',
-        backgroundSize: 'cover',
+        backgroundRepeat: 'no-repeat',
+        backgroundSize: 'min(60vh, 520px)',
       }}
     >
       <div style={{ width: '100%', maxWidth: 400 }}>