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

server/src/config/brand.js

@@ -31,7 +31,7 @@ const brand = {
   // Visual
   accent: process.env.BRAND_ACCENT_COLOR || '#7f99bd',
   logo: process.env.BRAND_LOGO || '', // empty → no logo image rendered
-  hero: process.env.BRAND_HERO || '/assets/img/hero-moon.png',
+  hero: process.env.BRAND_HERO || '/assets/img/runic-emblem.png',
   favicon: process.env.BRAND_FAVICON || '/assets/img/favicon.ico', // Runic Gateway default emblem
 }