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

server/src/config/brand.js

@@ -32,7 +32,7 @@ const brand = {
   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',
-  favicon: process.env.BRAND_FAVICON || '', // empty → no favicon link injected
+  favicon: process.env.BRAND_FAVICON || '/assets/img/favicon.ico', // Runic Gateway default emblem
 }
 
 // Discord embeds want an int (0xRRGGBB). Parse the accent hex once; fall back to