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/lib/labels.ts

@@ -50,7 +50,7 @@ export function stackLabel(stack: string): string {
 
 /**
  * Full URL for a service exposed at <subdomain>.<gateway.domain>. The domain is
- * derived from the dashboard's own host (it is served at castle-app.<domain>), so
+ * derived from the dashboard's own host (it is served at castle.<domain>), so
  * this returns null when the dashboard is on a bare host (off mode, no subdomains).
  */
 export function subdomainUrl(subdomain: string): string | null {