dev-deploy: enable Dev Sandbox bootstrap and synthetic-report loader

Two long-standing dev-environment ergonomics had not survived the
move from the bespoke local-dev stack to the CI-driven dev-deploy:

1. `BACKEND_DEV_SANDBOX_EMAIL` defaulted to an empty string in the
   dev-deploy compose, so the auto-provisioned "Dev Sandbox" game
   never appeared on `https://www.galaxy.lan`. Bake `dev@galaxy.lan`
   as the default — matches `.env.example` and lets a developer who
   logs in with that email find a ready-to-play game in the lobby.

2. The lobby's synthetic-report loader was gated on
   `import.meta.env.DEV`, which is true only for `vite dev` (the
   tools/local-dev path). The long-lived dev environment builds
   with `vite build` (production mode), so the section was always
   stripped from its bundle. Gate it on an explicit
   `VITE_GALAXY_DEV_AFFORDANCES` flag instead and set it both in
   `.env.development` (preserves `pnpm dev` behaviour) and in the
   `dev-deploy.yaml` build step. The `prod-build.yaml` build path
   leaves the flag unset, so production stays clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/frontend/.env.development

@@ -16,3 +16,9 @@ VITE_GATEWAY_BASE_URL=http://localhost:5173
 # key. Pairs with `tools/local-dev/keys/gateway-response.pem`. The pair
 # is dev-only — see `tools/local-dev/keys/README.md` before rotating.
 VITE_GATEWAY_RESPONSE_PUBLIC_KEY=nIG54tCuNiIKrazt8Hh7YxmmU/BhpseGhIIgj164Chw=
+
+# Opt in to dev-time UI affordances that should never reach a
+# production bundle — currently the synthetic-report loader in the
+# lobby. Mirror this flag in any long-lived dev build (e.g.
+# `dev-deploy.yaml`); the prod build path leaves it unset.
+VITE_GALAXY_DEV_AFFORDANCES=true