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/Switch.tsx

@@ -30,8 +30,11 @@ export function Switch({
       title={title}
       disabled={disabled}
       onClick={() => onChange(!checked)}
-      className={`relative inline-flex h-5 w-9 shrink-0 items-center rounded-full transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent/40 disabled:cursor-not-allowed disabled:opacity-50 ${
-        checked ? "bg-accent" : "bg-surface-4"
+      // On: the solid accent (white knob clears 3:1 in every theme). Off: a
+      // border-light track with an inset fg-muted/80 ring (≥3:1 in every theme), so the control's edge
+      // is visible on any surface (surface-4 alone was ~1:1 on a light card).
+      className={`relative inline-flex h-5 w-9 shrink-0 items-center rounded-full transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent focus-visible:ring-offset-1 focus-visible:ring-offset-surface-0 disabled:cursor-not-allowed disabled:opacity-50 ${
+        checked ? "bg-accent-solid" : "bg-border-light ring-1 ring-inset ring-fg-muted/80"
       }`}
     >
       <span