fix(ui): give every page a main landmark and mark the sidebars as navigation - #1587

No page of the interface carries a `main` landmark — `grep` for `<main` or
`role="main"` across `ui/src` and `ui/template` returns nothing. The sidebar
is a plain `div` as well.

Screen reader users navigate by landmarks. Without a `main`, there is no way
to skip past the header and the sidebar to the content; every visit to every
page starts by tabbing through the whole navigation again. WCAG 2.4.1
(Bypass Blocks) is the criterion, and a landmark is the least intrusive way
to satisfy it.

Both layouts that carry the sidebar are changed: the wrapper around `Outlet`
becomes `main`, and the sidebar container becomes `nav`. Class names are
untouched, so nothing moves.

Not included, and worth a look separately: the layouts that do not use the
sidebar (`Admin`, `Users/Settings`, `Legal`) still need one, and with two
`nav` landmarks on a page — this one and the header — an `aria-label` on each
would help. That needs a new i18n key, so it is left out of a change that is
otherwise markup only.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

The admin area has a layout of its own and sits directly under `pages/Layout`,
so the two changes above did not reach it. Nineteen further routes — signing
in, registering, recovering an account, the error pages — have no layout at
all; they get a pathless one that is nothing but a `main`.

`pages/Layout` itself would be the wrong place: it wraps the header, so a
landmark there would put the navigation inside the content region and "skip to
content" would land before it.

Verified structurally — 88 paths before, the same 88 after, no page removed —
and by building the front end and stepping through the affected routes: each
renders inside exactly one `main` and draws its content.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>