ci: retire the standalone web image and build-web step (#1444)

The gateway image now carries the SPA bundle, so web.Dockerfile,
scripts/build-web.mjs (+ test), and the publish build-web step go away.
The two CI-structure tests drop build-web from their expected-effects
lists. .env.example replaces the Next block with WEB_DIST_DIR docs.

scripts/publish-gate-structure.test.mjs

@@ -199,7 +199,6 @@ test('the real publish pipeline: a failed verify provably blocks every publish e
   assert.deepEqual(effects.sort(), [
     'build-appservice',
     'build-gateway',
-    'build-web',
     'publish-next-npm',
     'publish-npm',
   ]);