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>

web/index.html

@@ -9,13 +9,14 @@
 <link rel="icon" type="image/png" sizes="128x128" id="favicon" href="/favicon.png" data-light="/favicon.png" data-dark="/favicon-dark.png">
 <link rel="apple-touch-icon" href="/apple-touch-icon.png">
 <link rel="stylesheet" data-src="app.css">
+<script src="/boot.js"></script>
 </head>
 <body>
 <header id="topbar">
   <button class="iconbtn" id="burger" title="Feeds" aria-label="Feeds" data-icon="menu"></button>
   <!-- One group per thing acted on, in the order the panes read: feeds, then the selected item.
        A phone hides the item group, which it has no table for. -->
-  <span id="applogo" title="iPX {version}"><img class="logo onlight" src="/logo.svg" alt="iPX"><img class="logo ondark" src="/logo-dark.svg" alt="iPX"><img class="logo classic" src="/icon.png" alt="iPodderX"></span>
+  <span id="applogo" title="iPX"><img class="logo onlight" src="/logo.svg" alt="iPX"><img class="logo ondark" src="/logo-dark.svg" alt="iPX"><img class="logo classic" src="/icon.png" alt="iPodderX"></span>
   <div class="tgroup">
     <button id="addFeed" title="Add a feed" aria-label="Add a feed" data-icon="plus"></button>
     <button id="tbRemove" title="Unsubscribe from this feed" aria-label="Unsubscribe from this feed" data-icon="circleMinus" disabled></button>