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).

core/src/castle_core/generators/caddyfile.py

@@ -161,7 +161,7 @@ def _host_static_block(label: str, host: str, serve_dir: str) -> list[str]:
 # Castle's own control plane: the dashboard frontend and the API it calls. These
 # names are the subdomains they're published at in acme mode, and the pair served
 # on the :<port> site in off mode (no domain → no subdomains).
-_DASHBOARD = "castle-app"
+_DASHBOARD = "castle"
 _API = "castle-api"