Give staging its own address rather than a spare room in the main house

A subdomain means a separate origin, so a staging run can no longer overwrite
the localStorage the testteam is playing on. That makes the /staging alias
pointless, so the Dockerfile goes back to exactly what is deployed today.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

HANDOFF.md

@@ -23,11 +23,10 @@ the primary target.
   a push. **Bump `main.js?v=N` in index.html every time** or phones keep the old build.
   The Dockerfile must also COPY any new top-level file (it currently ships `index.html`,
   `manifest.webmanifest`, `*.js`, `assets/`).
-- **Staging**: the `staging` branch deploys to splatter-tennis.wuwi.one/staging (own Coolify
-  resource, same repo, branch `staging`). Test there first, then merge to `main` for the testteam.
-  The branch carries NO diff against main — the image serves the app at both `/` and `/staging/`,
-  so it can be fast-forwarded and merged freely. Note it shares the origin (and therefore
-  localStorage) with prod.
+- **Staging**: the `staging` branch deploys to its own subdomain (own Coolify resource, same repo,
+  branch `staging`, same Dockerfile). Test there first, then merge to `main` for the testteam.
+  The branch carries NO diff against main, so it fast-forwards and merges freely — keep it that way.
+  A subdomain, not a subfolder, so staging gets its own origin and can't stomp the prod localStorage.
 - **Oliver edits files himself** (tuning values, sometimes sprites). ALWAYS Read current file state
   before editing; never assume your last version is what is on disk.