Flatten proxy config to a bool (drop the caddy key)

CaddySpec had only `enable` left (path_prefix/host removed; extra_snippets was
unused), so `proxy: { caddy: {} }` was three levels of nesting for a checkbox —
and it invited drift (supabase silently reverted to proxy.caddy.host). Replace it
with a plain `ServiceSpec.proxy: bool`.

- Model: remove ProxySpec/CaddySpec; `proxy: bool = False`. Readers simplified to
  `bool(svc.proxy)` (generator, castle-api summaries, CLI info).
- CLI create / dashboard editor + create form: write `proxy: true` / a checkbox.
- Configs migrated to `proxy: true` (and supabase's stale proxy.caddy.host block
  removed).
- Docs (registry, dns-and-tls, design, stack guides) + tests/fixtures updated;
  also fixed lingering path-model staleness in the stack guides.

docs/stacks/react-vite.md

@@ -56,9 +56,9 @@ import { defineConfig } from "vite"
 import react from "@vitejs/plugin-react"
 
 export default defineConfig({
-  // Castle sets VITE_BASE to the gateway serve prefix at build time (`/<name>/`,
-  // or `/` for the root app). Reading it here makes the bundle's absolute asset
-  // URLs resolve when served behind the gateway at a subpath — no hand-tuning.
+  // Castle sets VITE_BASE=/ at build time (every frontend serves at its
+  // subdomain root). Reading it here keeps the bundle.s absolute asset
+  // URLs resolving at the root — no hand-tuning.
   base: process.env.VITE_BASE ?? "/",
   plugins: [react(), tailwindcss()],
   resolve: {
@@ -157,8 +157,7 @@ services:
     expose:
       http:
         internal: { port: 5173 }
-    proxy:
-      caddy: {}   # expose the dev server at my-frontend.<gateway.domain>
+    proxy: true   # expose the dev server at my-frontend.<gateway.domain>
 ```
 
 See @docs/registry.md for the full registry reference.