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/__tests__/i18n.test.ts

@@ -7,6 +7,17 @@
 import { describe, it, expect } from "vitest";
 import i18n from "i18next";
 
+describe("<html lang>", () => {
+  it("follows the resolved language on every change", async () => {
+    await i18n.changeLanguage("vi-VN");
+    expect(document.documentElement.lang).toBe("vi");
+    await i18n.changeLanguage("ko");
+    expect(document.documentElement.lang).toBe("en");
+    await i18n.changeLanguage("en");
+    expect(document.documentElement.lang).toBe("en");
+  });
+});
+
 describe("i18n resources", () => {
   it("should provide Vietnamese translations for navigation keys", async () => {
     await i18n.changeLanguage("vi");

client/src/i18n/index.ts

@@ -16,6 +16,8 @@
  * Unknown regional variants (e.g. `en-US`) map to supported base languages via
  * `nonExplicitSupportedLngs`.
  *
+ * `<html lang>` follows the resolved language on init and on every change.
+ *
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 /* =============================================================================
@@ -94,6 +96,15 @@ import splash_vi from "./locales/vi/splash.json";
 import lanes_en from "./locales/en/lanes.json";
 import lanes_vi from "./locales/vi/lanes.json";
 
+/** Keep `<html lang>` on the active language, so a screen reader speaks the
+ *  page with the right voice and the browser hyphenates/translates correctly.
+ *  Registered before `init`, which fires the first `languageChanged`. */
+export function syncDocumentLang(): void {
+  if (typeof document === "undefined") return;
+  document.documentElement.lang = i18n.resolvedLanguage || i18n.language || "en";
+}
+i18n.on("languageChanged", syncDocumentLang);
+
 i18n
   .use(LanguageDetector)
   .use(initReactI18next)
@@ -169,5 +180,7 @@ i18n
     },
   });
 
+syncDocumentLang();
+
 /** Configured i18next instance — import side effects run {@link init}. */
 export default i18n;

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"
 }

client/src/i18n/locales/vi/errors.json

@@ -28,5 +28,6 @@
     "glob": "Glob",
     "agent": "Agent",
     "other": "Khác"
-  }
+  },
+  "badResponse": "Máy chủ trả về phản hồi không mong đợi thay vì dữ liệu. Dashboard có thể đang khởi động lại, hoặc một proxy đang trả lời thay nó — hãy tải lại sau giây lát."
 }

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

@@ -68,5 +68,9 @@
     "workspace": "Workspace {{cost}}",
     "session": "Session {{cost}}"
   },
-  "version": "Phiên bản"
+  "version": "Phiên bản",
+  "openMenu": "Mở menu",
+  "closeMenu": "Đóng menu",
+  "sidebar": "Thanh bên",
+  "now": "bây giờ"
 }