fix(ci): set BASE_URL so pre-registrant invite route doesn't 500

invite.post.js requires process.env.BASE_URL to build the invite link,
returning 500 when unset. The CI workflow stubbed Resend / Mongo / JWT
but missed BASE_URL, so the admin-pre-registrants invite spec timed
out waiting for the success toast. Set BASE_URL to the test server's
URL on both jobs.

.forgejo/workflows/test.yml

@@ -30,6 +30,7 @@ jobs:
       NUXT_PUBLIC_COMING_SOON: 'false'
       NODE_ENV: development
       ALLOW_DEV_TEST_ENDPOINTS: 'true'
+      BASE_URL: http://localhost:3000
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
@@ -100,6 +101,7 @@ jobs:
       NUXT_PUBLIC_COMING_SOON: 'false'
       NODE_ENV: development
       ALLOW_DEV_TEST_ENDPOINTS: 'true'
+      BASE_URL: http://localhost:3000
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4