Rename the dashboard program castle-app -> castle

The dashboard is now served at castle.<domain> (was castle-app.<domain>). Since
the subdomain is the program name, this is just a program rename plus the one
special-case constant.

- Generator: _DASHBOARD = "castle" (the :9000 redirect target and off-mode root).
- Program file renamed (runtime): programs/castle-app.yaml -> programs/castle.yaml
  (source unchanged: repo:app).
- Dashboard UI: ProgramDetail shows a frontend's subdomain (via subdomainUrl),
  dropping the old castle-app-at-root special-case; comment refs updated.
- Docs: registry.md/design.md/react-vite.md updated to the subdomain model and the
  `castle` name (react-vite.md also corrected — frontends serve at their subdomain
  root with VITE_BASE=/, and builds are manual via `castle program build`, not
  deploy).

app/src/services/api/client.ts

@@ -1,5 +1,5 @@
 // Resolve the castle-api base URL. The gateway serves each service at its own
-// subdomain (<name>.<domain>), so when the dashboard runs at castle-app.<domain>
+// subdomain (<name>.<domain>), so when the dashboard runs at castle.<domain>
 // the API lives at castle-api.<domain> — a cross-origin call (castle-api allows
 // CORS *). When served at a bare host (dev, or the off-mode :9000 gateway), the
 // API is reachable same-origin at /api.