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/__tests__/StatusBar.test.tsx

@@ -61,17 +61,20 @@ describe("StatusBar", () => {
     const user = userEvent.setup();
     const onConnectionClick = vi.fn();
     at("/", <StatusBar wsConnected left="15rem" onConnectionClick={onConnectionClick} />);
-    expect(screen.getByText("Live")).toBeInTheDocument();
-    expect(screen.getByText(`v${__APP_VERSION__}`)).toBeInTheDocument();
+    const conn = screen.getByRole("button", { name: /Live/ });
+    expect(conn).toHaveTextContent(`v${__APP_VERSION__}`);
     expect(await screen.findByText("Today $1.23")).toBeInTheDocument();
     expect(screen.getByTestId("status-bar-counts").textContent).toBe("2 running · 1 need you");
-    await user.click(screen.getByText("Live"));
+    await user.click(conn);
     expect(onConnectionClick).toHaveBeenCalled();
+    // Polite live region: the connection and who is waiting, nothing that ticks.
+    expect(screen.getByRole("status")).toHaveTextContent("Live · 1 need you");
   });
 
   it("says Disconnected when the socket is down", () => {
     at("/", <StatusBar wsConnected={false} left="0" onConnectionClick={() => {}} />);
-    expect(screen.getByText("Disconnected")).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: /Disconnected/ })).toBeInTheDocument();
+    expect(screen.getByRole("status")).toHaveTextContent("Disconnected");
   });
 
   it("off the Workspace page shows no workspace or session cost", async () => {