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.

app/(app)/error.tsx

@@ -0,0 +1,47 @@
+"use client";
+
+import { AlertTriangle, RotateCcw } from "lucide-react";
+import Link from "next/link";
+import { useEffect } from "react";
+import { Button, LINK_BUTTON_CLASS } from "@/components/ui/Button";
+
+// Without this file a failed render drops the user on Next.js's own error
+// screen — no navigation, no way back, and in production just "a client-side
+// exception occurred". `reset()` re-renders the segment, which is enough for
+// the common case of a transient Supabase timeout.
+export default function AppError({ error, reset }: { error: Error & { digest?: string }; reset: () => void }) {
+  useEffect(() => {
+    console.error("Route error:", error);
+  }, [error]);
+
+  return (
+    <div className="mx-auto flex max-w-lg flex-col items-start gap-4 rounded border border-border bg-white p-6">
+      <span className="flex h-10 w-10 items-center justify-center rounded-full bg-danger-bg text-danger-text">
+        <AlertTriangle className="h-5 w-5" />
+      </span>
+      <div>
+        <h1 className="text-lg font-bold text-ink">Diese Ansicht konnte nicht geladen werden</h1>
+        <p className="mt-1 text-sm text-ink-body">
+          Die Daten wurden nicht verändert. Meist hilft ein erneuter Versuch; bleibt der Fehler, wenden Sie sich bitte an die
+          IT-Betreuung.
+        </p>
+      </div>
+      {/* The digest is the only handle on the server-side stack trace, which
+          is deliberately not sent to the browser in production. */}
+      {error.digest && (
+        <p className="rounded bg-surface px-3 py-2 font-mono text-xs text-ink-muted">
+          Fehlerkennung: {error.digest}
+        </p>
+      )}
+      <div className="flex flex-wrap gap-2">
+        <Button onClick={reset}>
+          <RotateCcw className="h-4 w-4" />
+          Erneut versuchen
+        </Button>
+        <Link href="/" className={LINK_BUTTON_CLASS}>
+          Zur Übersicht
+        </Link>
+      </div>
+    </div>
+  );
+}