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.

app/(app)/loading.tsx

@@ -0,0 +1,30 @@
+// Every page in this group is server-rendered per request (they all read
+// from Supabase), so without this the browser sits on the previous page with
+// no feedback until the server answers — on the employee list, long enough
+// to look broken.
+export default function Loading() {
+  return (
+    <div className="flex flex-col gap-4" role="status" aria-label="Wird geladen">
+      <div className="h-9 w-64 animate-pulse rounded bg-border-subtle" />
+      <div className="flex flex-wrap gap-3">
+        {[240, 160, 160].map((w, i) => (
+          <div key={i} style={{ width: w }} className="h-10 animate-pulse rounded bg-border-subtle" />
+        ))}
+      </div>
+      <div className="overflow-hidden rounded border border-border bg-white">
+        {Array.from({ length: 8 }, (_, i) => (
+          <div key={i} className="flex items-center gap-4 border-b border-border-subtle px-4 py-3 last:border-0">
+            <div className="h-9 w-9 shrink-0 animate-pulse rounded-full bg-border-subtle" />
+            <div className="flex flex-1 flex-col gap-1.5">
+              <div className="h-3.5 w-48 animate-pulse rounded bg-border-subtle" />
+              <div className="h-3 w-32 animate-pulse rounded bg-border-subtle" />
+            </div>
+            <div className="hidden h-3.5 w-24 animate-pulse rounded bg-border-subtle sm:block" />
+            <div className="hidden h-3.5 w-20 animate-pulse rounded bg-border-subtle md:block" />
+          </div>
+        ))}
+      </div>
+      <span className="sr-only">Daten werden geladen…</span>
+    </div>
+  );
+}