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/ui/SlideOver.tsx

@@ -1,7 +1,9 @@
 "use client";
 
 import { X } from "lucide-react";
-import { useEffect, type ReactNode } from "react";
+import { useId, useRef, type ReactNode } from "react";
+import { Button } from "./Button";
+import { useDialogFocus } from "./useDialogFocus";
 
 type SlideOverProps = {
   open: boolean;
@@ -13,37 +15,41 @@ type SlideOverProps = {
 };
 
 export function SlideOver({ open, onClose, title, subtitle, children, footer }: SlideOverProps) {
-  useEffect(() => {
-    if (!open) return;
-    const onKey = (e: KeyboardEvent) => {
-      if (e.key === "Escape") onClose();
-    };
-    window.addEventListener("keydown", onKey);
-    return () => window.removeEventListener("keydown", onKey);
-  }, [open, onClose]);
+  const dialogRef = useRef<HTMLDivElement>(null);
+  const titleId = useId();
+  useDialogFocus(open, onClose, dialogRef);
 
   return (
-    <div className={`fixed inset-0 z-50 ${open ? "pointer-events-auto" : "pointer-events-none"}`} aria-hidden={!open}>
+    // Stays mounted so the slide transition can run, which means a closed
+    // panel's fields are still in the tab order — aria-hidden does not remove
+    // them. `inert` does, and also blocks clicks, so several closed panels no
+    // longer pile up invisible tab stops at the end of the page.
+    <div className="fixed inset-0 z-50" inert={!open}>
       <div
         className={`absolute inset-0 bg-black/40 transition-opacity ${open ? "opacity-100" : "opacity-0"}`}
         onClick={onClose}
+        aria-hidden
       />
       <div
+        ref={dialogRef}
         role="dialog"
         aria-modal="true"
-        aria-label={title}
-        className={`absolute right-0 top-0 flex h-dvh w-full max-w-md flex-col bg-white shadow-xl transition-transform duration-200 ${
+        aria-labelledby={titleId}
+        tabIndex={-1}
+        className={`absolute right-0 top-0 flex h-dvh w-full max-w-md flex-col bg-white shadow-xl outline-none transition-transform duration-200 ${
           open ? "translate-x-0" : "translate-x-full"
         }`}
       >
         <div className="flex shrink-0 items-start justify-between border-b border-border px-4 py-3 pt-[max(0.75rem,env(safe-area-inset-top))] sm:px-6 sm:py-4">
           <div className="min-w-0">
-            <h2 className="text-lg font-bold text-ink">{title}</h2>
+            <h2 id={titleId} className="text-lg font-bold text-ink">
+              {title}
+            </h2>
             {subtitle && <p className="truncate text-sm text-ink-muted">{subtitle}</p>}
           </div>
-          <button type="button" onClick={onClose} aria-label="Schließen" className="-mr-1 rounded p-2 text-ink-muted hover:bg-surface">
+          <Button variant="icon" onClick={onClose} aria-label="Schließen" className="-mr-1">
             <X className="h-5 w-5" />
-          </button>
+          </Button>
         </div>
         <div className="flex-1 overflow-y-auto overscroll-contain px-4 py-4 sm:px-6">{children}</div>
         {footer && (