Say env var changes take effect at the next deploy or rollback (closes #255)

The hint under the app page's environment variable editor asked for a
container restart. Environment variables are set when a container is
created, and upaas's restart stops and starts the same container, so it
keeps the old values; deploy and rollback each create a new container.
The hint now says so in one sentence, styled with alert-warning like the
page's Volume Mounts note: its old text-amber-600 class is not in the
built stylesheet. A handler test renders the app page and checks the
hint.

Model: opus-5-5

TODO.md

@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-10-01: The hint under the app page's environment variable editor now says
+  changes take effect at the next deploy or rollback, in the page's warning
+  style, instead of asking for a container restart, which keeps the old values
+  (#255).
+
 - 2026-10-01: An app with no volume mounts says on its page, and in the log of
   each deploy, that a deploy or rollback loses the files it writes and a restart
   keeps them; the README's Volume mounts section says a deploy or rollback keeps