fix(ui): give the remaining pages a main landmark

[culfin]

Third and last follow-up to #1587 (after #1589 for the admin area). Nineteen
routes hang straight off pages/Layout with no layout of their own — signing
in, registering, recovering an account, confirming an address, and the error
pages. They were the ones left without a landmark.

pages/Layout itself is the wrong place to add one: it wraps the header, so a
main there would put the navigation inside the content region, and "skip to
content" would land before it rather than after.

The change

A pathless layout route that is nothing but a main:

const Index: FC = () => (
  <main>
    <Outlet />
  </main>
);

Three of them, one per stretch of such routes, so every path, guard and index
stays exactly where it was.

Verified structurally: 88 paths before, the same 88 after, no page removed,
three layout nodes added.

The diff looks larger than it is. Read with git diff -w:

 ui/src/pages/PlainLayout/index.tsx | 43 ++++++++++++
 ui/src/router/routes.ts            | 24 +++++++

24 added lines in the router — three eight-line wrappers. Everything else is
one indentation level.

Verified by running it

Built the front end and stepped through the affected routes in a browser:

route	<main>	renders
/users/login	1	"Welcome to Answer"
/users/register	1	"Welcome to Answer"
/50x	1	the error page
unknown path	1	the 404 page

The console errors in that run come from the API being absent in a static
build — custom.css is served by the Go side, not by the bundle.

With this, #1587, #1589 and this one together, every route of the interface
sits inside exactly one main.

🤖 Generated with Claude Code

[culfin]

Folded into #1587, which now covers every route rather than a third of them. Nothing is dropped; the commits are on that branch. Closing to keep the queue readable.