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/TerminatePanel.tsx

@@ -3,6 +3,8 @@
 import { useRouter } from "next/navigation";
 import { useState } from "react";
 import { terminateEmployee } from "@/actions/employees";
+import { Button } from "@/components/ui/Button";
+import { SelectField, TextField, TextareaField } from "@/components/ui/Field";
 import { SlideOver } from "@/components/ui/SlideOver";
 import { useToast } from "@/components/ui/Toast";
 import type { Database } from "@/lib/supabase/types";
@@ -53,16 +55,12 @@ export function TerminatePanel({ open, onClose, employee, directReportCount }: T
       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-danger-solid px-4 py-2 text-sm font-semibold text-white disabled:opacity-50"
-          >
+          </Button>
+          <Button variant="danger" onClick={handleSubmit} pending={pending}>
             Austritt bestätigen
-          </button>
+          </Button>
         </>
       }
     >
@@ -72,31 +70,16 @@ export function TerminatePanel({ open, onClose, employee, directReportCount }: T
             {directReportCount} direkte Berichte werden automatisch der nächsthöheren Führungskraft zugeordnet.
           </div>
         )}
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Austrittsdatum*</label>
-          <input
-            type="date"
-            value={exitDate}
-            onChange={(e) => setExitDate(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">Beendigungsart</label>
-          <select value={reason} onChange={(e) => setReason(e.target.value)} className="w-full rounded border border-border px-3 py-2 text-sm">
-            {EXIT_REASONS.map((r) => (
-              <option key={r} value={r}>
-                {r}
-              </option>
-            ))}
-          </select>
-        </div>
-        <div>
-          <label className="mb-1 block text-sm font-semibold text-ink">Anmerkung</label>
-          <textarea value={note} onChange={(e) => setNote(e.target.value)} rows={3} className="w-full rounded border border-border px-3 py-2 text-sm" />
-        </div>
-        <div>
-          <p className="mb-2 text-sm font-semibold text-ink">Offboarding-Checkliste</p>
+        <TextField label="Austrittsdatum" required type="date" value={exitDate} onChange={setExitDate} />
+        <SelectField
+          label="Beendigungsart"
+          value={reason}
+          onChange={setReason}
+          options={EXIT_REASONS.map((r) => ({ value: r, label: r }))}
+        />
+        <TextareaField label="Anmerkung" rows={3} value={note} onChange={setNote} />
+        <fieldset>
+          <legend className="mb-2 text-sm font-semibold text-ink">Offboarding-Checkliste</legend>
           <div className="flex flex-col gap-2">
             {CHECKLIST_ITEMS.map((item, i) => (
               <label key={item} className="flex items-center gap-2 text-sm text-ink-body">
@@ -109,7 +92,7 @@ export function TerminatePanel({ open, onClose, employee, directReportCount }: T
               </label>
             ))}
           </div>
-        </div>
+        </fieldset>
       </div>
     </SlideOver>
   );