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/styles/theme.css

@@ -1009,6 +1009,47 @@ button[disabled] {
   border-color: #8a4b47;
 }
 
+/* Site footer: powered-by badge left-justified, info centered. */
+.site-footer-inner {
+  position: relative;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 16px;
+  max-width: 1100px;
+  margin: 0 auto;
+}
+.site-footer-badge {
+  height: 56px;
+  width: auto;
+  flex: 0 0 auto;
+  opacity: 0.85;
+}
+.site-footer-info {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 6px;
+  text-align: center;
+}
+/* Pin the badge to the left while the info column stays visually centered. */
+@media (min-width: 641px) {
+  .site-footer-badge {
+    position: absolute;
+    left: 0;
+    top: 50%;
+    transform: translateY(-50%);
+  }
+}
+@media (max-width: 640px) {
+  .site-footer-inner {
+    flex-direction: column;
+  }
+  .site-footer-badge {
+    height: 44px;
+  }
+}
+
 /* Draft-preview banner on the public renderer. */
 .page-preview-banner {
   border: 1px solid var(--accent);