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

[culfin]

No page of the interface carries a main landmark, and the sidebars are plain
divs.

Visible on this project's own instance

meta.answer.dev/questions, fetched with a Googlebot user agent:

$ curl -s -A "…Googlebot…" https://meta.answer.dev/questions | grep -c "<main"
0

Same in the source:

$ grep -rn "<main\|role=\"main\"" ui/src ui/template | wc -l
0

Why it matters

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. That is WCAG 2.4.1
(Bypass Blocks), and a landmark is the least intrusive way to satisfy it.

The change

Every route now sits inside exactly one main:

layout	covers
SideNavLayout, SideNavLayoutWithoutFooter	the forum, tags, users, badges
Admin	28 admin routes
new PlainLayout (pathless)	19 routes with no layout of their own — signing in, registering, account recovery, the error pages

pages/Layout would have been the wrong place for a single landmark: it wraps
the header, so main there would put the navigation inside the content region
and "skip to content" would land before it.

The sidebar containers become nav in the same three files. Class names are
untouched, so nothing moves on screen.

Verified

• Structurally: 88 paths before, the same 88 after, no page removed, three
  layout nodes added.
• By running it: built the front end and stepped through /users/login,
  /users/register, /50x and an unknown path — each renders inside exactly
  one main and draws its content.
• prettier, eslint, tsc.

The router diff looks larger than it is. Read with git diff -w it is 24
added lines; everything else is one indentation level.

Deliberately not included

With two nav landmarks on a page — this one and the header — an aria-label
on each would tell them apart. That needs a new i18n key across 45 language
files, which does not belong in a change that is otherwise markup only.

Supersedes #1589 and #1591, which were the same work split across three pull
requests.

🤖 Generated with Claude Code

[robinv8]

Thanks for consolidating this into one PR.

The SPA change looks right: one main per layout, sidebars as nav, and a pathless PlainLayout so login / error pages get a landmark without putting the header inside main. Plugin routes hang off SideNavLayout, so they pick this up too.

Two gaps worth confirming:

1. Server-rendered HTML is unchanged. ui/template/header.html still has <nav id="header"> and <div id="pcSideNav">, and the content slot is not a main. The curl/Googlebot check in the description would still print 0 after this lands, because that path never goes through React. Screen-reader users who run JS are covered; crawlers and no-JS first paint are not. If the goal is only the hydrated UI, please say so in the description. If the curl case is in scope, the Go templates need the same landmarks.

2. pages/Layout replaces <Outlet /> with HttpErrorContent when httpStatusCode is set, so API-driven 403/50x never reach the layout main.

The missing aria-label on the two navs is fine as a follow-up (agreed that 45 locale files do not belong here).

No objection to landing the SPA markup as-is once (1) is explicit.