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/PromotePanel.tsx

@@ -3,6 +3,8 @@
 import { useRouter } from "next/navigation";
 import { useState } from "react";
 import { promoteEmployee } 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, PaygradeType } from "@/lib/supabase/types";
@@ -56,47 +58,26 @@ export function PromotePanel({ open, onClose, employee }: { open: boolean; onClo
       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-purple-text px-4 py-2 text-sm font-semibold text-white disabled:opacity-50"
-          >
+          </Button>
+          {/* Promotion keeps its own purple, matching the Beförderung badge
+              used in the history tab. */}
+          <Button onClick={handleSubmit} pending={pending} className="!bg-purple-text text-white hover:brightness-110">
             Befördern
-          </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">Neue Position*</label>
-          <input value={newTitle} onChange={(e) => setNewTitle(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">Paygrade</label>
-          <select
-            value={paygrade}
-            onChange={(e) => setPaygrade(e.target.value as PaygradeType)}
-            className="w-full rounded border border-border px-3 py-2 text-sm"
-          >
-            {PAYGRADES.map((p) => (
-              <option key={p.value} value={p.value}>
-                {p.label}
-              </option>
-            ))}
-          </select>
-        </div>
+        <TextField label="Wirksam ab" required type="date" value={effectiveDate} onChange={setEffectiveDate} />
+        <TextField label="Neue Position" required value={newTitle} onChange={setNewTitle} />
+        <SelectField
+          label="Paygrade"
+          value={paygrade}
+          onChange={(v) => setPaygrade(v as PaygradeType)}
+          options={PAYGRADES}
+        />
       </div>
     </SlideOver>
   );