Give the Admin tab sub-sections of its own

Administration was one scrolling page with three cards on it: the teams,
the people, and the settings. There was no way to link somebody to the
settings, no way back to the top of the user list but scrolling, and the
poll loop refetched all three endpoints every tick however little of the
page you were looking at.

Each is now a route -- /admin/teams, /admin/users, /admin/settings --
reached from a strip across the top, with /admin an overview that says
how many of each there are. The three cards themselves are untouched;
they are simply rendered one at a time, so a tab fetches only what it
shows. The Users page is the exception and fetches the teams too, since
its invite form has to offer a team to invite somebody into.

The strip is ordinary links rather than chips. Chips filter what a page
already shows, here and in the queue, and these four go somewhere: the
browser's Back walks them, a reload lands where you were, and the click
is intercepted by the same handler every other link in the app uses.
The current one is marked with aria-current="page", the convention the
tab bar has used since it existed, so the state lives on the attribute
and not in a class.

admin.js owns the table of the four routes, because it also builds the
strip that links to them; app.js parses against that table rather than
keeping a second list to drift from it. Adding a fifth sub-section is
one line.

The bottom tab bar still has six items. 56b8191 made it count-agnostic
when Admin arriving pushed it past four, and the note there records that
six at 420px already leaves 55-65px each -- so the sub-sections went
inside the Admin page rather than beside it.

A person's page keeps its own route at /admin/users/{id}; its back link
now returns to the user list rather than to the top of everything.

Nobody has looked at this in a browser, the same caveat ac9af8e carried.
What is checked is the wiring: the module graph evaluates at every admin
URL, all four tabs render against live server responses with one
aria-current each and the fetches the table above describes, the
non-administrator branch still refuses without fetching, and the server
serves index.html for each new path so a reload survives. The strip's
appearance at phone and desktop width is not checked.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

internal/web/static/js/adminuser.js

@@ -83,7 +83,7 @@ function render() {
 }
 
 function backLink() {
-  return h('a', { class: 'back-link', href: '/admin' }, icon('chevronLeft'), h('span', { text: 'Admin' }));
+  return h('a', { class: 'back-link', href: '/admin/users' }, icon('chevronLeft'), h('span', { text: 'Users' }));
 }
 
 // --- identity --------------------------------------------------------------
@@ -271,7 +271,7 @@ async function deleteUser() {
     return;
   }
   toast('User deleted.');
-  navigate('/admin');
+  navigate('/admin/users');
 }
 
 // --- plumbing --------------------------------------------------------------