make test-frontend runs against a stack of its own

The suite creates and deletes flows, dashboards and users, and ran against
whatever stack was up, which is the dev instance's data. make test-stack starts
the compose project fluksio-test (compose.test.yml over the local stack's
files) with its own containers, volumes and image tags, served by the local
Traefik on app.test.localhost and api.test.localhost, and leaves it running.
The hostname is fixed, so it no longer comes off the running dev stack, and a
LAN stack makes no difference. The suite's default origin is the test stack.

README.md

@@ -66,7 +66,7 @@ make dev-frontend  # Vite on :5173
 ```
 
 ```sh
-make test           # pytest + Playwright (the e2e half needs the stack up)
+make test           # pytest + Playwright (the e2e half on its own stack, behind make dev's Traefik)
 make lint           # ruff + mypy + biome
 make generate-client  # regenerate the frontend SDK from the OpenAPI schema
 ```