A separate admin page: server settings, accounts and the log

/admin, with Server, Accounts and Log sections chosen by the URL's hash. The
server sends the page and /admin.js to admins only (anyone else asking for the
page goes back to the app, and the script is 403), and removes the header's link
to it from everyone else's page rather than hiding it. The API keeps refusing
all of it to non-admins as before.

Settings becomes personal: theme, OPML import and export, and the schedule and
download folder to read. The server fields, the Users dialog and the Log dialog
move out of dialogs.ts into admin.ts.

The CSS moves out of index.html into web/app.css, which both pages load as
/app.css?v=<hash>, served immutable like the scripts. The smoke test checks both
pages.

Closes #19.

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

docs/architecture.md

@@ -19,7 +19,9 @@ it to a running daemon.
 | `src/auth.rs` | Argon2id hashing, session tokens, header names | — |
 | `src/web.rs` | axum: HTTP API, auth, SSE, media streaming | — |
 | `src/logbuf.rs` | Ring buffer behind the UI's Log view | — |
-| `web/index.html` | The page's markup and CSS | — |
+| `web/index.html` | The app's markup | — |
+| `web/admin.html` | The admin page's markup: server settings, accounts, the log. Sent to admins only | — |
+| `web/app.css` | The stylesheet both pages share | — |
 | `web/src/*.ts` | The page's script, one scope split across files, type-checked by `npx tsc` | — |
 | `web/build.mjs` | swc: strips the types into `app.js`/`login.js`, named in the page by a hash of their contents, and minifies | — |
 | `build.rs` | Runs `web/build.mjs` into `OUT_DIR`, where `web.rs` `include_str!`s the result | — |