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/i18n/locales/en/errors.json

@@ -28,5 +28,6 @@
     "glob": "Glob",
     "agent": "Agent",
     "other": "Other"
-  }
+  },
+  "badResponse": "The server sent an unexpected response instead of data. The dashboard may be restarting, or a proxy is answering in its place — reload in a moment."
 }

client/src/i18n/locales/en/nav.json

@@ -68,5 +68,9 @@
     "workspace": "Workspace {{cost}}",
     "session": "Session {{cost}}"
   },
-  "version": "Version"
+  "version": "Version",
+  "openMenu": "Open menu",
+  "closeMenu": "Close menu",
+  "sidebar": "Sidebar",
+  "now": "now"
 }