ipx-web: the page in its own container, behind Caddy (#203)

web/build.mjs builds a static site, web/dist, with the icons and typeface
beside the pages; Dockerfile.web serves it with Caddy, which passes /api
and /media to ipx on the same origin, so cookies, live updates and
Cloudflare Access work as before. ipx serves no page any more: build.rs
and every include_str! of the web files are gone, and its image is built
without node.

What ipx did to the page as it served it is kept another way. Caddy asks
ipx before serving a page (forward_auth): /api/page-check sends a browser
not signed in to /login and a non-admin away from /admin, and refuses them
/admin.js; /api/login-check sends someone Access vouches for past the
form; a page asked for with ?token= goes to /api/signin, which sets the
cookie. The theme and the admin link, once written into the page per
person, are set by web/boot.js in <head> from the theme cookie and the
last visit, before the first frame, and /api/me brings the version and
the account's theme from before.

Caddy strips the Cf-Access-* headers from every peer but TUNNEL_PEER, the
address cloudflared's requests come from, which is the boundary
trusted_proxies drew; ipx then trusts only Caddy's address.

The browser suite runs Caddy in front of its daemon, so it tests what
production would run. Not deployed: production's cutover is on #203.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile.web

@@ -0,0 +1,13 @@
+# ipx-web (#203): the frontend, built from web/ and served by Caddy, which passes the API to ipx.
+# Built from the repo's root: docker buildx build -f Dockerfile.web .
+FROM node:22-slim AS build
+WORKDIR /src
+COPY package.json package-lock.json ./
+RUN npm ci --omit=dev
+COPY web ./web
+RUN node web/build.mjs /dist
+
+FROM caddy:2
+COPY web/Caddyfile /etc/caddy/Caddyfile
+COPY --from=build /dist /srv
+EXPOSE 8080