ci(a11y): axe-core gate + accessibility-tree snapshots (#222)

e2e/a11y.spec.ts runs @axe-core/playwright (WCAG 2.x A/AA) over the login
page, room timeline + composer, message options menu, thread panel, user
settings and room settings, failing on critical/serious findings other
than colour contrast (reported, not gated: generated avatar colours and
portal false positives). Aria snapshots of the composer, message menu,
thread panel and settings nav catch lost names/roles/live regions.

Burned down what the first run found:
- NavItem: callers' aria-selected is not valid on a div (axe critical);
  it now drives data-selected for styling and aria-current="page".
- Composer placeholder at 0.5 opacity was ~2.3:1; now P300.
- Voice-limit and explore custom-limit number inputs had no label.
- Thread panel is an <aside aria-label="Thread">; the settings modal is a
  role=dialog; the settings sections are a <nav>; the message action
  menu carries data-message-menu + a label.

Also allows WebKit's CI wording for the well-known probe ("Could not
connect … Connection refused") that failed run #2003.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/settings/Settings.tsx

@@ -153,7 +153,7 @@ export function Settings({ initialPage, requestClose }: SettingsProps) {
             </PageNavHeader>
             <Box grow="Yes" direction="Column">
               <PageNavContent>
-                <div style={{ flexGrow: 1 }}>
+                <nav aria-label="Settings sections" style={{ flexGrow: 1 }}>
                   {menuItems.map((item) => (
                     <MenuItem
                       key={item.name}
@@ -174,7 +174,7 @@ export function Settings({ initialPage, requestClose }: SettingsProps) {
                       </Text>
                     </MenuItem>
                   ))}
-                </div>
+                </nav>
               </PageNavContent>
               <Box style={{ padding: config.space.S200 }} shrink="No" direction="Column">
                 <UseStateProvider initial={false}>