Subdomain-only gateway routing; drop path prefixes

Collapse the two proxy shapes (path_prefix + host) to a single checkbox: a
service is either port-only, or exposed at <service-name>.<gateway.domain>. Path
routes and their prefix-stripping foot-guns are gone; the subdomain is always the
service name (rename the service to change it).

- Schema: CaddySpec is just `enable` (presence = expose). service_proxy_targets
  returns (expose, port, base_url); Deployment carries `subdomain` (was
  proxy_path/proxy_host). Registry/deploy/mesh/CLI updated in lockstep.
- Generator: compute_routes emits one host route per exposed service/frontend
  (address = name). acme mode = one *.<domain> site with a reverse_proxy matcher
  per service + a file_server matcher per frontend; the :<port> site redirects to
  the dashboard. off mode = a HTTP control plane on :<port> (dashboard at / +
  /api → castle-api); other services are port-only.
- Dashboard/API: castle-app and castle-api are ordinary subdomains. The dashboard
  derives the API base at runtime (castle-api.<domain> on a subdomain, else /api)
  and calls it cross-origin — castle-api already allows CORS *. Frontends build
  with VITE_BASE=/ (served at their subdomain root).
- CLI: `service create` drops --path/--host; --no-proxy makes it port-only.
- Docs/tests reworked for the model; docs use generic placeholders only (no
  personal host/domain/IP details).

core/src/castle_core/stacks.py

@@ -71,11 +71,10 @@ async def _run(
 def _vite_base(name: str) -> str:
     """The base path a castle-served static frontend builds against.
 
-    Matches the Caddyfile serve prefix: castle-app is the root app ('/'); every
-    other static frontend mounts at '/<name>/'. Exposed to the build as VITE_BASE
-    so the bundle's absolute asset URLs resolve at the gateway subpath (the
-    vite.config reads `base: process.env.VITE_BASE ?? '/'`)."""
-    return "/" if name == "castle-app" else f"/{name}/"
+    Every frontend now serves at the **root of its own subdomain**
+    (`<name>.<gateway.domain>`), so the base is always '/'. Exposed to the build
+    as VITE_BASE (the vite.config reads `base: process.env.VITE_BASE ?? '/'`)."""
+    return "/"
 
 
 def _source_dir(comp: ProgramSpec, root: Path) -> Path: