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/hire/StepPosition.tsx

@@ -1,4 +1,6 @@
 import { X } from "lucide-react";
+import { Button } from "@/components/ui/Button";
+import { Field, SelectField, TextField } from "@/components/ui/Field";
 import { Lookup } from "@/components/ui/Lookup";
 import { fmtDate } from "@/lib/format";
 import type { OpenPositionResolved } from "@/lib/positions";
@@ -20,23 +22,32 @@ export function StepPosition({ draft, update, openPositions }: StepPositionProps
 
   return (
     <div className="flex flex-col gap-4">
-      <div>
-        <label className="mb-1 block text-sm font-semibold text-ink">Position*</label>
-        {!selected ? (
-          <Lookup<OpenPositionResolved>
-            placeholder="Positionsname oder -nummer…"
-            onSearch={search}
-            onSelect={(p) => update({ positionId: p.id })}
-            renderResult={(p) => (
-              <div>
-                <div className="font-semibold text-ink">{p.title}</div>
-                <div className="text-xs text-ink-muted">
-                  {p.position_number} · {p.orgLabel}
+      {!selected ? (
+        <Field
+          label="Position"
+          required
+          hint={openPositions.length === 0 ? "Derzeit sind keine offenen Positionen vorhanden." : undefined}
+        >
+          {(p) => (
+            <Lookup<OpenPositionResolved>
+              {...p}
+              placeholder="Positionsname oder -nummer…"
+              onSearch={search}
+              onSelect={(pos) => update({ positionId: pos.id })}
+              renderResult={(pos) => (
+                <div>
+                  <div className="font-semibold text-ink">{pos.title}</div>
+                  <div className="text-xs text-ink-muted">
+                    {pos.position_number} · {pos.orgLabel}
+                  </div>
                 </div>
-              </div>
-            )}
-          />
-        ) : (
+              )}
+            />
+          )}
+        </Field>
+      ) : (
+        <div>
+          <p className="mb-1 block text-sm font-semibold text-ink">Position</p>
           <div className="flex items-center justify-between rounded border border-border bg-surface p-3">
             <div>
               <div className="text-sm font-semibold text-ink">{selected.title}</div>
@@ -45,31 +56,32 @@ export function StepPosition({ draft, update, openPositions }: StepPositionProps
               </div>
               <div className="text-xs text-ink-muted">Gültig ab {fmtDate(selected.valid_from)}</div>
             </div>
-            <button type="button" onClick={() => update({ positionId: "" })} aria-label="Auswahl aufheben">
-              <X className="h-4 w-4 text-ink-muted" />
-            </button>
+            <Button variant="icon" onClick={() => update({ positionId: "" })} aria-label="Auswahl aufheben">
+              <X className="h-4 w-4" />
+            </Button>
           </div>
-        )}
-        {openPositions.length === 0 && <p className="mt-1 text-xs text-ink-muted">Derzeit sind keine offenen Positionen vorhanden.</p>}
-      </div>
+        </div>
+      )}
 
-      <div>
-        <label className="mb-1 block text-sm font-semibold text-ink">Besetzung*</label>
-        <select
-          value={draft.besetzung}
-          onChange={(e) => update({ besetzung: e.target.value as HireDraftData["besetzung"] })}
-          className="w-full rounded border border-border px-3 py-2 text-sm"
-        >
-          <option value="">Bitte wählen…</option>
-          <option value="Extern">Extern</option>
-          <option value="Intern">Intern</option>
-        </select>
-      </div>
+      <SelectField
+        label="Besetzung"
+        required
+        value={draft.besetzung}
+        onChange={(v) => update({ besetzung: v as HireDraftData["besetzung"] })}
+        placeholder="Bitte wählen…"
+        options={[
+          { value: "Extern", label: "Extern" },
+          { value: "Intern", label: "Intern" },
+        ]}
+      />
 
-      <div>
-        <label className="mb-1 block text-sm font-semibold text-ink">Führungskraft</label>
-        <input value={selected?.managerName ?? "–"} disabled className="w-full rounded border border-border bg-surface px-3 py-2 text-sm text-ink-muted" />
-      </div>
+      <TextField
+        label="Führungskraft"
+        value={selected?.managerName ?? "–"}
+        onChange={() => {}}
+        disabled
+        hint="Ergibt sich aus der gewählten Position."
+      />
     </div>
   );
 }