Add integrity checks for lockfiles

I haven't done any frontend dev in a long time, so the lockfiles are
generated with old npm. Regenerate (without updating) so we won't need
to fetch integrity checks at runtime.

    $ node --version
    v16.16.0
    $ npm --version
    8.11.0
    $ npm install --package-lock-only
    $ npm install --prefix=frontend/ --package-lock-only

Separately, I should really get onto node 20 (or at least 18), but doing
that would require me to solve the webpack/openssl issue in the Vue and
I'd rather just consider investing my effort into using Vite instead.

.tool-versions

@@ -1,3 +1,3 @@
 python 3.10.14
 poetry 1.8.3
-nodejs 16.16.0
+nodejs 16.20.2