feat(a11y): contrast tokens, accent-solid, responsive drawer, keyboard menus

- Contrast: --fg-muted reaches 4.5:1 on every surface in all nine themes
  (dark default was 2.55:1); status chips and accent text clear 4.5:1 on
  their own tints. New --accent-solid(/-hover) + tailwind `accent-solid`
  is the fill behind white text (.btn-primary, Switch on, Checkbox tick);
  --accent is now a text colour. Switch off / unchecked Checkbox get a
  >=3:1 edge.
- <html lang> follows the chosen language; a global prefers-reduced-motion
  rule; index.html body uses a token instead of text-gray-200.
- Layout: the sidebar starts collapsed below lg and becomes an off-canvas
  drawer below md (menu button, backdrop, Escape, useDialog).
- ContextMenu: focus moves in, arrows/Home/End, Escape/Tab close, focus
  returns, a keyboard-opened menu anchors to its target. Select is a real
  listbox button (aria-expanded, activedescendant, type-ahead). PageTabs
  implements APG tabs with a tabpanel. New .btn-secondary/.btn-danger/
  .btn-icon and a .select class for native selects, all with focus rings.
- StatusBar announces connection and "N need you" politely; Sidebar's
  connection modal uses useDialog; EmptyState/Skeleton/AiToolIcon a11y.
- api.ts request(): a non-JSON 200 throws a translated EBADRESPONSE
  instead of "Unexpected token '<'".

client/src/components/EmptyState.tsx

@@ -91,7 +91,7 @@ export function EmptyState({ icon: Icon, title, description, action, compact }:
       <div
         className={`rounded-2xl bg-surface-4 flex items-center justify-center ${compact ? "w-10 h-10 mb-3" : "w-14 h-14 mb-5"}`}
       >
-        <Icon className="w-6 h-6 text-fg-muted" />
+        <Icon className="w-6 h-6 text-fg-muted" aria-hidden />
       </div>
       <h3 className="text-base font-medium text-fg-secondary mb-2">{title}</h3>
       <p className={`text-sm text-fg-muted max-w-md ${compact ? "" : "mb-6"}`}>{description}</p>