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.

src/app/components/editor/Editor.css.ts

@@ -67,7 +67,9 @@ export const EditorTextarea = style([
 export const EditorPlaceholderContainer = style([
   DefaultReset,
   {
-    opacity: config.opacity.Placeholder,
+    // [Gitea #222] folds' Placeholder opacity (0.5) lands at ~2.3:1 on the
+    // composer surface; P300 keeps it visibly secondary at AA contrast.
+    opacity: config.opacity.P300,
     pointerEvents: 'none',
     userSelect: 'none',
   },

src/app/components/nav/NavItem.tsx

@@ -9,16 +9,35 @@ export const NavItem = as<
   {
     highlight?: boolean;
   } & css.RoomSelectorVariants
->(({ as: AsNavItem = 'div', className, highlight, variant, radii, children, ...props }, ref) => (
-  <AsNavItem
-    className={classNames(css.NavItem({ variant, radii }), className)}
-    data-highlight={highlight}
-    {...props}
-    ref={ref}
-  >
-    {children}
-  </AsNavItem>
-));
+>(
+  (
+    {
+      as: AsNavItem = 'div',
+      className,
+      highlight,
+      variant,
+      radii,
+      children,
+      'aria-selected': selected,
+      ...props
+    },
+    ref,
+  ) => (
+    // [Gitea #222] Callers pass `aria-selected`, but that attribute is only
+    // valid on option/tab/row roles; on a plain div axe flags it as critical.
+    // Keep the prop for callers and styling, expose the state as aria-current.
+    <AsNavItem
+      className={classNames(css.NavItem({ variant, radii }), className)}
+      data-highlight={highlight}
+      data-selected={selected === true || selected === 'true' ? true : undefined}
+      aria-current={selected === true || selected === 'true' ? 'page' : undefined}
+      {...props}
+      ref={ref}
+    >
+      {children}
+    </AsNavItem>
+  ),
+);
 
 export const NavLink = forwardRef<HTMLAnchorElement, ComponentProps<typeof Link>>(
   ({ className, ...props }, ref) => (

src/app/components/nav/styles.css.ts

@@ -69,7 +69,7 @@ const NavItemBase = style({
     [`&:has(.${NavLink}:active)`]: {
       backgroundColor: ContainerActive,
     },
-    '&[aria-selected=true]': {
+    '&[data-selected=true]': {
       backgroundColor: ContainerActive,
     },
     [`&:has(.${NavLink}:focus-visible)`]: {