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

e2e/a11y.spec.ts-snapshots/composer-editor.aria.yml

@@ -0,0 +1,2 @@
+- textbox "Send a message...":
+    - paragraph: Send a message...

e2e/a11y.spec.ts-snapshots/composer-send.aria.yml

@@ -0,0 +1,2 @@
+- button "Send message":
+    - img

e2e/a11y.spec.ts-snapshots/message-menu.aria.yml

@@ -0,0 +1,39 @@
+- button "Add Reaction":
+    - text: ''
+    - img
+- button "Reply":
+    - text: ''
+    - img
+- button "Forward":
+    - text: ''
+    - img
+- button "Bookmark Message":
+    - text: ''
+    - img
+- button "Remind Me":
+    - text: ''
+    - img
+- button "Edit Message":
+    - text: ''
+    - img
+- button "Read Receipts":
+    - text: ''
+    - img
+- button "Copy Text":
+    - text: ''
+    - img
+- button "Translate":
+    - text: ''
+    - img
+- button "Copy Link":
+    - text: ''
+    - img
+- button "Copy Lotus Link":
+    - text: ''
+    - img
+- button "Pin Message":
+    - text: ''
+    - img
+- button "Delete":
+    - text: ''
+    - img

e2e/a11y.spec.ts-snapshots/settings-nav.aria.yml

@@ -0,0 +1,22 @@
+- navigation "Settings sections":
+    - button "General" [pressed]:
+        - img
+        - paragraph: General
+    - button "Account":
+        - img
+        - paragraph: Account
+    - button "Notifications":
+        - img
+        - paragraph: Notifications
+    - button "Devices":
+        - img
+        - paragraph: Devices
+    - button "Emojis & Stickers":
+        - img
+        - paragraph: Emojis & Stickers
+    - button "Developer Tools":
+        - img
+        - paragraph: Developer Tools
+    - button "About":
+        - img
+        - paragraph: About

e2e/a11y.spec.ts-snapshots/thread-panel.aria.yml

@@ -0,0 +1,31 @@
+- complementary "Thread":
+    - paragraph: Thread
+    - paragraph: A11y Room
+    - button "Thread notifications":
+        - img
+    - button "Close thread":
+        - img
+    - log "Thread timeline":
+        - article:
+            - button /e2e_a11y_\w+, open profile/:
+                - img
+            - button /e2e_a11y_\w+/
+            - time: /\d+:\d+ (AM|PM)/
+            - text: thread root for a11y
+        - paragraph: 1 reply
+        - article:
+            - button /e2e_a11y_\w+, open profile/:
+                - img
+            - button /e2e_a11y_\w+/
+            - time: /\d+:\d+ (AM|PM)/
+            - text: a reply
+    - button "More actions":
+        - img
+    - textbox "Send a message...":
+        - paragraph: Send a message...
+    - button "Insert sticker":
+        - img
+    - button "Insert emoji":
+        - img
+    - button "Send message":
+        - img