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/Picklist.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { X } from "lucide-react";
+import { CONTROL_CLASS } from "./Field";
 
 // Dropdown-to-add, chip-to-remove multi-select for short, fixed option
 // lists (no search needed) — e.g. academic titles. The dropdown only offers
@@ -11,11 +12,16 @@ export function Picklist({
   value,
   onChange,
   placeholder = "Hinzufügen…",
+  // Forwarded onto the <select> so a surrounding <Field> can label it.
+  id,
+  "aria-describedby": describedBy,
 }: {
   options: string[];
   value: string[];
   onChange: (value: string[]) => void;
   placeholder?: string;
+  id?: string;
+  "aria-describedby"?: string;
 }) {
   const available = options.filter((o) => !value.includes(o));
 
@@ -27,12 +33,14 @@ export function Picklist({
     <div className="flex flex-col gap-2">
       <select
         key={value.length}
+        id={id}
+        aria-describedby={describedBy}
         defaultValue=""
         onChange={(e) => {
           if (e.target.value) onChange([...value, e.target.value]);
         }}
         disabled={available.length === 0}
-        className="w-full rounded border border-border px-3 py-2 text-sm disabled:bg-surface disabled:text-ink-muted"
+        className={CONTROL_CLASS}
       >
         <option value="" disabled>
           {available.length > 0 ? placeholder : "Alle Optionen ausgewählt"}