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/tailwind.config.js

@@ -38,6 +38,12 @@ const tinted = {
     hover: tint("--accent-hover"),
     muted: "color-mix(in srgb, rgb(var(--accent)) 15%, rgb(var(--tint-base)))",
   },
+  // The fill behind white text (`.btn-primary`, Switch on, Checkbox tick):
+  // `--accent` is a TEXT colour, too light in dark themes to carry white.
+  "accent-solid": {
+    DEFAULT: tint("--accent-solid"),
+    hover: tint("--accent-solid-hover"),
+  },
   status: {
     success: tint("--status-success"),
     danger: tint("--status-danger"),
@@ -69,6 +75,10 @@ export default {
           hover: "rgb(var(--accent-hover) / <alpha-value>)",
           muted: "rgb(var(--accent) / 0.15)",
         },
+        "accent-solid": {
+          DEFAULT: "rgb(var(--accent-solid) / <alpha-value>)",
+          hover: "rgb(var(--accent-solid-hover) / <alpha-value>)",
+        },
         fg: {
           primary: "rgb(var(--fg-primary) / <alpha-value>)",
           secondary: "rgb(var(--fg-secondary) / <alpha-value>)",