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/EmployeeDetail.tsx

@@ -4,6 +4,7 @@ import { ArrowLeft, ArrowRightLeft, Clock, Pencil, RotateCcw, TrendingUp, XCircl
 import Link from "next/link";
 import { useState } from "react";
 import { Avatar } from "@/components/ui/Avatar";
+import { Button } from "@/components/ui/Button";
 import { StatusChip } from "@/components/ui/StatusChip";
 import { fmtFullName, tenure } from "@/lib/format";
 import type { Database } from "@/lib/supabase/types";
@@ -97,31 +98,34 @@ export function EmployeeDetail(props: EmployeeDetailProps) {
             )}
             {canEditData && <ActionButton icon={Pencil} label="Daten ändern" onClick={() => setPanel("daten")} />}
             {isActive && (
-              <button
+              <Button
+                variant="secondary"
+                size="sm"
                 onClick={() => setPanel("terminate")}
-                className="flex items-center gap-1.5 rounded border border-danger-solid px-3 py-1.5 text-sm font-semibold text-danger-solid hover:bg-danger-bg"
+                className="!border-danger-solid !text-danger-solid hover:!bg-danger-bg"
               >
                 <XCircle className="h-4 w-4" /> Austritt
-              </button>
+              </Button>
             )}
             {employee.status === "Ausgetreten" && (
-              <button
-                onClick={() => setPanel("rehire")}
-                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={() => setPanel("rehire")}>
                 <RotateCcw className="h-4 w-4" /> Wiedereinstellen
-              </button>
+              </Button>
             )}
           </div>
         </div>
       </div>
 
-      <div className="flex gap-1 border-b border-border">
+      {/* Tabs, so the list gets the tablist role and each button says
+          whether it is the selected one. */}
+      <div role="tablist" aria-label="Mitarbeiterdetails" className="flex gap-1 overflow-x-auto border-b border-border">
         {TABS.map((t) => (
           <button
             key={t}
+            role="tab"
+            aria-selected={tab === t}
             onClick={() => setTab(t)}
-            className={`border-b-2 px-4 py-2 text-sm font-semibold ${
+            className={`whitespace-nowrap border-b-2 px-4 py-2 text-sm font-semibold focus-visible:outline-2 focus-visible:outline-offset-[-2px] focus-visible:outline-brand-500 ${
               tab === t ? "border-brand-500 text-brand-700" : "border-transparent text-ink-muted hover:text-ink"
             }`}
           >
@@ -166,8 +170,8 @@ export function EmployeeDetail(props: EmployeeDetailProps) {
 
 function ActionButton({ icon: Icon, label, onClick }: { icon: typeof ArrowRightLeft; label: string; onClick: () => void }) {
   return (
-    <button onClick={onClick} className="flex items-center gap-1.5 rounded border border-border px-3 py-1.5 text-sm font-semibold text-ink-body hover:bg-surface">
+    <Button variant="secondary" size="sm" onClick={onClick}>
       <Icon className="h-4 w-4" /> {label}
-    </button>
+    </Button>
   );
 }