docs(ui): correct the synthetic-report loader gate comment

Stage 2 of the dev-as-prod-mirror rework. The legacy-report (synthetic)
report loader is already available in the dev-deploy UI: it is gated by
the build-time flag VITE_GALAXY_DEV_AFFORDANCES (set "true" in
dev-deploy.yaml line 89, unset in prod-build.yaml so prod strips it),
not by import.meta.env.DEV. Correct the stale header comment that
claimed import.meta.env.DEV. No functional change — the desired
"loader in dev, absent in prod" posture already holds.

ui/frontend/src/api/synthetic-report.ts

@@ -1,6 +1,8 @@
 // DEV-only synthetic-report loader. Backs the "Load synthetic report"
-// affordance on the lobby (visible behind `import.meta.env.DEV`) and
-// the in-game shell layout's bypass for the synthetic game id range.
+// affordance on the lobby (visible when the build-time flag
+// `VITE_GALAXY_DEV_AFFORDANCES === "true"` — the dev and dev-deploy
+// bundles; stripped from prod) and the in-game shell layout's bypass
+// for the synthetic game id range.
 //
 // The accepted JSON shape mirrors `pkg/model/report.Report` as
 // emitted by `tools/local-dev/legacy-report/cmd/legacy-report-to-json`.