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

package.json

@@ -107,6 +107,7 @@
     "workbox-precaching": "7.4.1"
   },
   "devDependencies": {
+    "@axe-core/playwright": "4.13.0",
     "@lotusguild/element-call-embedded": "0.25.0-lotus.9",
     "@playwright/test": "1.63.0",
     "@rollup/plugin-inject": "5.0.5",