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.

components/shell/NewHireButton.tsx

@@ -2,18 +2,18 @@
 
 import { Plus } from "lucide-react";
 import { useHireWizard } from "@/components/hire/HireWizardContext";
+import { Button } from "@/components/ui/Button";
 
 export function NewHireButton() {
   const { openWizard } = useHireWizard();
 
   return (
-    <button
-      type="button"
-      onClick={() => openWizard()}
-      className="flex items-center gap-1.5 rounded bg-brand-500 px-3 py-1.5 text-sm font-semibold text-white hover:bg-brand-600"
-    >
+    <Button size="sm" onClick={() => openWizard()}>
       <Plus className="h-4 w-4" />
-      Neueinstellung
-    </button>
+      {/* The label is the first thing worth dropping on a narrow bar; the
+          icon plus the accessible name carry it from there. */}
+      <span className="hidden sm:inline">Neueinstellung</span>
+      <span className="sr-only sm:hidden">Neueinstellung</span>
+    </Button>
   );
 }