ci: retire the standalone web image and build-web step (#1444)

The gateway image now carries the SPA bundle, so web.Dockerfile,
scripts/build-web.mjs (+ test), and the publish build-web step go away.
The two CI-structure tests drop build-web from their expected-effects
lists. .env.example replaces the Next block with WEB_DIST_DIR docs.

.env.example

@@ -40,9 +40,12 @@ BETTER_AUTH_SECRET=change-me-to-a-random-32-char-string
 BETTER_AUTH_URL=http://localhost:14242
 
 
-# ─── Web App (Next.js) ───────────────────────────────────────────────────────
-# Public gateway URL — accessible from the browser, not just the server.
-NEXT_PUBLIC_GATEWAY_URL=http://localhost:14242
+# ─── Web App (SPA) ───────────────────────────────────────────────────────────
+# Directory holding the built SPA bundle (vite build output). When set, the
+# gateway serves the SPA same-origin; when unset (dev), run the Vite dev
+# server (pnpm --filter @mosaicstack/web dev), which proxies to the gateway.
+# safe-default: unset in dev — SPA serving is an opt-in production concern
+#WEB_DIST_DIR=apps/web/dist
 
 
 # ─── OpenTelemetry ───────────────────────────────────────────────────────────