feat(brand): BRAND_* env scheme — instance branding without a rebuild

Replace baked-in UOM/MysticMoon/UOMysticmoon branding with a BRAND_* env
scheme so one prebuilt image runs as any shard; UOMysticmoon becomes the
first tenant that sets these vars rather than a special case in the code.

Architecture (chosen because the app ships as a prebuilt image):
- server/src/config/brand.js + bot/src/brand.js read BRAND_* once at boot,
  with Runic Gateway defaults.
- Text/colors reach the SPA at RUNTIME through the existing public settings
  API (settings.model.getPublic -> SiteContext), so no client rebuild. The
  admin-editable site title + contact email still override BRAND_NAME/email.
- SiteContext applies BRAND_ACCENT_COLOR to the --accent CSS var at runtime.
- Express templates the built index.html <title>/description/OG/favicon at
  serve time from BRAND_* (renderIndexHtml in app.js).
- Server-side consumers read brand directly: emails, TOTP issuer, API docs,
  boot logs, HTML error page. Bot uses it for embed color + logs.

Assets: logo/hero/favicon delivered from a ./brand:/app/brand bind-mount
(BRAND_LOGO/HERO/FAVICON), with neutral defaults baked in; hero falls back
to a built-in image when unset.

Scope: also genericized package.json names (uomysticmoon-* -> runic-gateway-*)
and the DB_NAME/DB_USER/COOKIE_NAME code defaults (runic_gateway/runic/
rg_token). Production keeps its real values by pinning them in .env — see
.env.uomysticmoon.example, which reproduces the exact UOMysticmoon identity
(proof the substitution works). Changing a deployed COOKIE_NAME invalidates
existing sessions, so UOMysticmoon pins uomm_token.

Verified: 193 server tests pass, client builds, app.js loads + templates the
built index.html, brand transform injects title/description/OG/favicon.

client/src/routes/admin/AdminLayout.jsx

@@ -126,7 +126,7 @@ const navBtnBase = {
 
 export default function AdminLayout() {
   const { user, logout } = useAuth()
-  const { mode } = useSite()
+  const { mode, siteTitle } = useSite()
   const navigate = useNavigate()
   const location = useLocation()
   const title =
@@ -225,7 +225,7 @@ export default function AdminLayout() {
           <MoonDot />
           <div>
             <div className="display" style={{ fontSize: '1.02rem', color: 'var(--head)', letterSpacing: '0.03em' }}>
-              UOMysticmoon
+              {siteTitle}
             </div>
             <div className="sans" style={{ color: 'var(--dim)', fontSize: '0.66rem', letterSpacing: '0.14em', textTransform: 'uppercase' }}>
               Admin