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/contexts/SiteContext.jsx

@@ -40,7 +40,7 @@ export function SiteProvider({ children }) {
     siteTitle: brand.name || settings.site_title || 'Runic Gateway',
     siteShortName: brand.shortName || brand.name || settings.site_title || 'Runic Gateway',
     contactEmail: brand.contactEmail || settings.contact_email || '',
-    heroImage: brand.hero || '/assets/img/hero-moon.png',
+    heroImage: brand.hero || '/assets/img/runic-emblem.png',
   }
 
   return <SiteContext.Provider value={value}>{children}</SiteContext.Provider>