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/TitleFields.tsx

@@ -1,5 +1,6 @@
 "use client";
 
+import { Field } from "@/components/ui/Field";
 import { Picklist } from "@/components/ui/Picklist";
 import { TITLE_PREFIXES, TITLE_SUFFIXES } from "@/lib/titles";
 
@@ -11,14 +12,12 @@ export type TitleValue = { titlePrefix: string[]; titleSuffix: string[] };
 export function TitleFields({ value, onChange }: { value: TitleValue; onChange: (patch: Partial<TitleValue>) => void }) {
   return (
     <div className="flex flex-col gap-3">
-      <div>
-        <label className="mb-1 block text-xs font-semibold text-ink-muted">Titel (vorangestellt)</label>
-        <Picklist options={TITLE_PREFIXES} value={value.titlePrefix} onChange={(titlePrefix) => onChange({ titlePrefix })} />
-      </div>
-      <div>
-        <label className="mb-1 block text-xs font-semibold text-ink-muted">Titel (nachgestellt)</label>
-        <Picklist options={TITLE_SUFFIXES} value={value.titleSuffix} onChange={(titleSuffix) => onChange({ titleSuffix })} />
-      </div>
+      <Field label="Titel (vorangestellt)" dense>
+        {(p) => <Picklist {...p} options={TITLE_PREFIXES} value={value.titlePrefix} onChange={(titlePrefix) => onChange({ titlePrefix })} />}
+      </Field>
+      <Field label="Titel (nachgestellt)" dense>
+        {(p) => <Picklist {...p} options={TITLE_SUFFIXES} value={value.titleSuffix} onChange={(titleSuffix) => onChange({ titleSuffix })} />}
+      </Field>
     </div>
   );
 }