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/components/Modal500.tsx

@@ -24,6 +24,8 @@ export function Modal500({ requestClose, children }: Modal500Props) {
           <Modal
             size="500"
             variant="Background"
+            role="dialog"
+            aria-modal="true"
             // On mobile expand to fill the viewport. On desktop fall back to the
             // folds `size="500"` width (~50rem) — overriding maxWidth here would
             // squish the two-pane settings layout.