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/employees/panels/TransferPanel.tsx

@@ -3,6 +3,8 @@
 import { useRouter } from "next/navigation";
 import { useMemo, useState } from "react";
 import { transferEmployee } from "@/actions/employees";
+import { Button } from "@/components/ui/Button";
+import { SelectField, TextField } from "@/components/ui/Field";
 import { SlideOver } from "@/components/ui/SlideOver";
 import { useToast } from "@/components/ui/Toast";
 import type { Database } from "@/lib/supabase/types";
@@ -66,67 +68,42 @@ export function TransferPanel({ open, onClose, employee, divisions, departments,
       subtitle={`${employee.first_name} ${employee.last_name} · ${employee.job_title}`}
       footer={
         <>
-          <button onClick={onClose} className="rounded px-4 py-2 text-sm font-semibold text-ink-body hover:bg-surface">
+          <Button variant="ghost" onClick={onClose}>
             Abbrechen
-          </button>
-          <button
-            onClick={handleSubmit}
-            disabled={pending}
-            className="rounded bg-brand-500 px-4 py-2 text-sm font-semibold text-white disabled:opacity-50"
-          >
+          </Button>
+          <Button onClick={handleSubmit} pending={pending}>
             Versetzen
-          </button>
+          </Button>
         </>
       }
     >
       <div className="flex flex-col gap-4">
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Wirksam ab*</label>
-          <input
-            type="date"
-            value={effectiveDate}
-            onChange={(e) => setEffectiveDate(e.target.value)}
-            className="w-full rounded border border-border px-3 py-2 text-sm"
-          />
-        </div>
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Neuer Bereich*</label>
-          <select
-            value={divisionId}
-            onChange={(e) => {
-              setDivisionId(e.target.value);
-              setTeamId("");
-            }}
-            className="w-full rounded border border-border px-3 py-2 text-sm"
-          >
-            {divisions.map((d) => (
-              <option key={d.id} value={d.id}>
-                {d.name}
-              </option>
-            ))}
-          </select>
-        </div>
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Neues Team*</label>
-          <select value={teamId} onChange={(e) => setTeamId(e.target.value)} className="w-full rounded border border-border px-3 py-2 text-sm">
-            <option value="">Bitte wählen…</option>
-            {teamsInDivision.map((t) => (
-              <option key={t.id} value={t.id}>
-                {t.name}
-              </option>
-            ))}
-          </select>
-        </div>
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Neuer Titel (optional)</label>
-          <input
-            value={newTitle}
-            onChange={(e) => setNewTitle(e.target.value)}
-            placeholder={employee.job_title}
-            className="w-full rounded border border-border px-3 py-2 text-sm"
-          />
-        </div>
-        <p className="text-xs text-ink-muted">Die neue Führungskraft wird automatisch anhand des Zielteams bestimmt.</p>
+        <TextField label="Wirksam ab" required type="date" value={effectiveDate} onChange={setEffectiveDate} />
+        <SelectField
+          label="Neuer Bereich"
+          required
+          value={divisionId}
+          onChange={(v) => {
+            setDivisionId(v);
+            setTeamId("");
+          }}
+          options={divisions.map((d) => ({ value: d.id, label: d.name }))}
+        />
+        <SelectField
+          label="Neues Team"
+          required
+          value={teamId}
+          onChange={setTeamId}
+          placeholder="Bitte wählen…"
+          options={teamsInDivision.map((t) => ({ value: t.id, label: t.name }))}
+        />
+        <TextField
+          label="Neuer Titel (optional)"
+          value={newTitle}
+          onChange={setNewTitle}
+          placeholder={employee.job_title}
+          hint="Die neue Führungskraft wird automatisch anhand des Zielteams bestimmt."
+        />
       </div>
     </SlideOver>
   );