Form primitives, keyboard-operable comboboxes, dialog focus, route states

Accessibility work on the UI layer, all of it rooted in one structural gap:
there were no form primitives, so every field was hand-assembled and every
field got the same details wrong.

Form primitives
- components/ui/Field.tsx (Field/TextField/SelectField/TextareaField) and
  Button.tsx. Field generates the control id with useId and derives htmlFor
  from it, which is what makes the association impossible to omit rather
  than merely conventional.
- 92 labels existed, 4 used htmlFor, and no input carried an id at all: a
  screen reader announced an unnamed edit box and clicking a label focused
  nothing. Now every label resolves to its control (0 unassociated), and the
  input class chain that appeared verbatim 85 times appears zero times.
- Field also takes a render prop, so Lookup, CountryPicker and Picklist get
  the same wiring instead of a second, partial solution.
- SearchInput replaces three hand-rolled copies of the icon-in-a-box search
  whose input had only a placeholder — not a label — and killed its own
  focus ring with outline-none and nothing in its place.
- Toggle groups (workdays, reorg change type) became fieldsets with
  aria-pressed; colour alone was carrying the selected state.

Comboboxes
- Lookup and CountryPicker were text inputs with a div of clickable buttons
  underneath: typeable, but no keyboard path to a result and nothing telling
  a screen reader a list had appeared. Both now carry role=combobox,
  aria-expanded/controls/activedescendant and listbox semantics, with arrow
  keys, Enter and Escape. Escape stops propagation, or it would close the
  surrounding dialog along with the dropdown.

Dialogs
- useDialogFocus centralises what Modal and SlideOver each owed the
  keyboard and neither provided beyond Escape: focus into the dialog on
  open, Tab and Shift+Tab cycling within it, focus restored to the trigger
  on close.
- SlideOver stays mounted for its transition, and aria-hidden does not
  remove anything from the tab order — so every closed panel was leaving
  invisible tab stops at the end of the page. `inert` fixes that.

Route states
- loading.tsx, error.tsx, not-found.tsx and global-error.tsx. Every page in
  the (app) group is server-rendered per request, so without loading.tsx a
  navigation showed nothing at all until the server answered, and a render
  error dropped the user on Next's own screen with no way back.

Tests
- 22 component tests (vitest jsdom project). Two of them found limits of the
  environment rather than of the code: jsdom implements neither `inert` nor
  scrollIntoView, so the inert test asserts the attribute and the missing
  scrollIntoView — which was taking the whole render down from inside an
  effect — is stubbed in the setup file.

tests/components/dialog-a11y.test.tsx

@@ -0,0 +1,130 @@
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { useState } from "react";
+import { describe, expect, it, vi } from "vitest";
+import { Button } from "@/components/ui/Button";
+import { TextField } from "@/components/ui/Field";
+import { Modal } from "@/components/ui/Modal";
+import { SlideOver } from "@/components/ui/SlideOver";
+
+function ModalHarness({ onClose = () => {} }: { onClose?: () => void }) {
+  const [first, setFirst] = useState("");
+  const [second, setSecond] = useState("");
+  return (
+    <Modal
+      open
+      onClose={onClose}
+      title="Testdialog"
+      footer={
+        <Button onClick={() => {}}>Speichern</Button>
+      }
+    >
+      <TextField label="Erstes Feld" value={first} onChange={setFirst} />
+      <TextField label="Zweites Feld" value={second} onChange={setSecond} />
+    </Modal>
+  );
+}
+
+describe("Modal focus management", () => {
+  it("names the dialog from its visible heading", () => {
+    render(<ModalHarness />);
+    expect(screen.getByRole("dialog", { name: "Testdialog" })).toBeInTheDocument();
+  });
+
+  it("moves focus into the dialog on open", () => {
+    render(<ModalHarness />);
+    // Not the page behind it: without this, the first Tab would start from
+    // the top of the document.
+    expect(document.body.contains(document.activeElement)).toBe(true);
+    expect(screen.getByRole("dialog").contains(document.activeElement)).toBe(true);
+  });
+
+  it("keeps Tab inside the dialog and wraps at the end", async () => {
+    render(<ModalHarness />);
+    const dialog = screen.getByRole("dialog");
+    for (let i = 0; i < 8; i++) {
+      await userEvent.tab();
+      expect(dialog.contains(document.activeElement)).toBe(true);
+    }
+  });
+
+  it("wraps backwards on Shift+Tab", async () => {
+    render(<ModalHarness />);
+    const dialog = screen.getByRole("dialog");
+    for (let i = 0; i < 8; i++) {
+      await userEvent.tab({ shift: true });
+      expect(dialog.contains(document.activeElement)).toBe(true);
+    }
+  });
+
+  it("closes on Escape", async () => {
+    const onClose = vi.fn();
+    render(<ModalHarness onClose={onClose} />);
+    await userEvent.keyboard("{Escape}");
+    expect(onClose).toHaveBeenCalled();
+  });
+
+  it("returns focus to the element that opened it", async () => {
+    function Toggle() {
+      const [open, setOpen] = useState(false);
+      return (
+        <>
+          <button onClick={() => setOpen(true)}>Öffnen</button>
+          <Modal open={open} onClose={() => setOpen(false)} title="Testdialog">
+            <p>Inhalt</p>
+          </Modal>
+        </>
+      );
+    }
+    render(<Toggle />);
+    const trigger = screen.getByRole("button", { name: "Öffnen" });
+    await userEvent.click(trigger);
+    expect(screen.getByRole("dialog")).toBeInTheDocument();
+    await userEvent.keyboard("{Escape}");
+    expect(trigger).toHaveFocus();
+  });
+});
+
+describe("SlideOver", () => {
+  it("marks a closed panel inert", () => {
+    // It stays mounted for the slide transition, so without `inert` its
+    // fields stay in the tab order behind the page — aria-hidden does not
+    // remove them.
+    //
+    // Asserted on the attribute rather than by tabbing: jsdom does not
+    // implement inert at all ("inert" in HTMLElement.prototype === false),
+    // so a focus-based assertion here would be testing jsdom, not this
+    // component. Browsers enforce it.
+    const { container } = render(
+      <SlideOver open={false} onClose={() => {}} title="Geschlossen">
+        <button>Im Panel</button>
+      </SlideOver>
+    );
+    expect(container.firstElementChild).toHaveAttribute("inert");
+  });
+
+  it("is not inert while open", () => {
+    const { container } = render(
+      <SlideOver open onClose={() => {}} title="Offen">
+        <button>Im Panel</button>
+      </SlideOver>
+    );
+    expect(container.firstElementChild).not.toHaveAttribute("inert");
+  });
+
+  it("traps focus once open", async () => {
+    render(
+      <>
+        <button>Dahinter</button>
+        <SlideOver open onClose={() => {}} title="Offen" footer={<Button>Speichern</Button>}>
+          <button>Im Panel</button>
+        </SlideOver>
+      </>
+    );
+    const dialog = screen.getByRole("dialog", { name: "Offen" });
+    for (let i = 0; i < 6; i++) {
+      await userEvent.tab();
+      expect(dialog.contains(document.activeElement)).toBe(true);
+    }
+  });
+});