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.

docker/web.Dockerfile

@@ -1,24 +0,0 @@
-FROM node:22-alpine AS base
-ENV PNPM_HOME="/pnpm"
-ENV PATH="$PNPM_HOME:$PATH"
-RUN corepack enable
-
-FROM base AS builder
-WORKDIR /app
-COPY pnpm-workspace.yaml pnpm-lock.yaml package.json ./
-COPY apps/web/package.json ./apps/web/
-COPY packages/ ./packages/
-# the root prepare script runs scripts/install-hooks.mjs on install
-COPY scripts/ ./scripts/
-RUN pnpm install --frozen-lockfile
-COPY . .
-RUN pnpm --filter @mosaicstack/web build
-
-FROM base AS runner
-WORKDIR /app
-ENV NODE_ENV=production
-COPY --from=builder /app/apps/web/.next/standalone ./
-COPY --from=builder /app/apps/web/.next/static ./apps/web/.next/static
-COPY --from=builder /app/apps/web/public ./apps/web/public
-EXPOSE 3000
-CMD ["node", "apps/web/server.js"]