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/wiki/Wiki.jsx

@@ -5,6 +5,7 @@ import PageHeader from '../../components/PageHeader.jsx'
 import { Loading, ErrorState, EmptyState } from '../../components/PageState.jsx'
 import { useAsync } from '../../lib/useAsync.js'
 import { api } from '../../api/client.js'
+import { useSite } from '../../contexts/SiteContext.jsx'
 
 function SearchBox({ initial, onSubmit }) {
   const [term, setTerm] = useState(initial || '')
@@ -61,6 +62,7 @@ function PageCard({ page }) {
 }
 
 export default function Wiki() {
+  const { siteShortName } = useSite()
   const [searchParams, setSearchParams] = useSearchParams()
   const activeCategory = searchParams.get('category')
   const activeTag = searchParams.get('tag')
@@ -92,7 +94,7 @@ export default function Wiki() {
         <PageHeader
           center
           eyebrow="Knowledge base"
-          title="Mysticmoon Wiki"
+          title={`${siteShortName} Wiki`}
           lead="A calm starting point for shard guides, the world and its lore, gameplay systems, and community rules."
         />
         <SearchBox initial={activeQ || ''} onSubmit={runSearch} />