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/AngehoerigeSection.tsx

@@ -4,6 +4,7 @@ import { Plus, Trash2 } from "lucide-react";
 import { useRouter } from "next/navigation";
 import { useState } from "react";
 import { deleteEmployeeDependent } from "@/actions/employees";
+import { Button } from "@/components/ui/Button";
 import { useToast } from "@/components/ui/Toast";
 import { fmtDate, todayIso } from "@/lib/format";
 import type { Database } from "@/lib/supabase/types";
@@ -38,13 +39,9 @@ export function AngehoerigeSection({ employeeId, dependents, effectiveDate }: {
       <div className="mb-3 flex items-center justify-between">
         <h3 className="text-xs font-bold uppercase tracking-wide text-brand-700">Angehörige</h3>
         <div className="flex items-center gap-4">
-          <button
-            type="button"
-            onClick={() => setModalOpen(true)}
-            className="flex items-center gap-1 text-xs font-semibold text-brand-700 hover:text-brand-600"
-          >
+          <Button variant="ghost" size="sm" onClick={() => setModalOpen(true)} className="!px-1 text-brand-700 hover:!bg-transparent hover:underline">
             <Plus className="h-3.5 w-3.5" /> Hinzufügen
-          </button>
+          </Button>
           <span className="text-xs text-ink-muted">{dependents.length} Personen</span>
         </div>
       </div>
@@ -73,15 +70,15 @@ export function AngehoerigeSection({ employeeId, dependents, effectiveDate }: {
                   <td className="px-4 py-2.5 text-ink-body">{d.sv_nummer ?? "–"}</td>
                   <td className="px-4 py-2.5 text-ink-body">{fmtDate(d.birth_date)}</td>
                   <td className="px-4 py-2.5 text-right">
-                    <button
-                      type="button"
+                    <Button
+                      variant="icon"
                       onClick={() => handleDelete(d.id)}
-                      disabled={deletingId === d.id}
-                      aria-label="Angehörige:n entfernen"
-                      className="text-ink-muted hover:text-danger-solid disabled:opacity-50"
+                      pending={deletingId === d.id}
+                      aria-label={`${d.first_name} ${d.last_name} als Angehörige:n entfernen`}
+                      className="hover:!text-danger-solid"
                     >
                       <Trash2 className="h-3.5 w-3.5" />
-                    </button>
+                    </Button>
                   </td>
                 </tr>
               ))}