feat(frontend): render Summary rail section + provider selector

New "Summary" rail section between the URL/Preview controls and .meta-list.
A completed summary renders as bold tl;dr, body paragraph, tag chips, and a
provider · model footer; missing or failed shows Generate; pending/running
shows an inline spinner and polls GET every 1500 ms until terminal.

- api.js: fetchEntrySummary + requestEntrySummary. The POST helper unwraps
  ApiError's { "error": ... } body so the missing-env-var message reaches
  the user verbatim rather than as a bare status code.
- ContextRail.jsx: state seeds from detail.latest_summary so the section
  renders immediately on selection. Polling is anchored on the summary
  status rather than started inside the click handler, so a job still
  running when the user navigates away and back is picked up again. A
  transient poll failure is swallowed — the next tick retries, and a real
  failure arrives as status === 'failed'.
- Regenerate passes force:true only when a completed summary is already
  shown; otherwise the request can take the server's 200 cache-hit path.
- Provider choice persists in sessionStorage under archivr:summary:provider,
  with try/catch around both accessors for private-mode browsers.
- Public sessions never see the selector or the Generate button, and the
  section renders at all only when a completed summary made it through the
  server's visibility gate.
- styles.css: .rail-summary-* only; spacing and the action button reuse
  .rail-section and .rail-rearchive-btn. The spinner honours
  prefers-reduced-motion — the text alone conveys the state.
- AGENTS.md: document the summary env vars alongside the existing
  external-tool convention.

Smoke-tested end to end against a scratch archive with a seeded markdown
entry: claude_cli produced a real summary (pending → running → completed in
~11s); a local mock server exercised the openai_compatible transport and
confirmed the Bearer header, model, and system/user role split on the wire;
unconfigured providers return 400 naming the exact variable; a video entry
returns 400 "v1 unsupported"; a repeat POST returns 200 from cache without
adding a row.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/archivr-server/static/index.html

@@ -6,8 +6,8 @@
     <title>Archivr</title>
     <link rel="icon" type="image/svg+xml" href="/favicon.svg">
     <link rel="icon" type="image/x-icon" href="/favicon.ico">
-    <script type="module" crossorigin src="/assets/index-B67momER.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-DicH9MNh.css">
+    <script type="module" crossorigin src="/assets/index-DdftptO_.js"></script>
+    <link rel="stylesheet" crossorigin href="/assets/index-e69aZgiQ.css">
   </head>
   <body>
     <div id="root"></div>