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>

CHANGELOG.md

@@ -27,6 +27,9 @@ The long form, with what was wrong before and how it was found, is in
 
 ### Changed
 
+- The server's settings, the accounts and the log are on their own admin page, /admin, reached by
+  the wrench in the header. Only an admin is sent the page, its script, or the link to it.
+  Settings is now yours alone: your theme and your subscriptions.
 - A feed that fails to check gets a red exclamation mark in the feed list, in the margin where a
   folder's triangle sits, and its page says why, in place of a pop-up per failure that everyone
   saw during a scan of every feed. A folder holding a failing feed has its triangle turn red.