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/SvNummerField.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { useState } from "react";
+import { TextField } from "@/components/ui/Field";
 import { formatSvnr, requiresAustrianSvnr, svnrErrorMessage, validateSvnr } from "@/lib/svnr";
 
 type SvNummerFieldProps = {
@@ -10,7 +11,7 @@ type SvNummerFieldProps = {
   locationCountry: string | null | undefined;
   /** ISO yyyy-mm-dd; enables the cross-check against the TTMMJJ tail. */
   birthDate?: string | null;
-  labelClassName?: string;
+  dense?: boolean;
 };
 
 /**
@@ -18,42 +19,28 @@ type SvNummerFieldProps = {
  * same rule. Errors are shown only once the field has been left, so the
  * message does not flash while the ten digits are still being typed.
  */
-export function SvNummerField({ value, onChange, locationCountry, birthDate, labelClassName }: SvNummerFieldProps) {
+export function SvNummerField({ value, onChange, locationCountry, birthDate, dense }: SvNummerFieldProps) {
   const [touched, setTouched] = useState(false);
   const applies = requiresAustrianSvnr(locationCountry);
   const error = applies && value.trim() !== "" ? validateSvnr(value, birthDate) : null;
-  const showError = touched && error !== null;
 
   return (
-    <div>
-      <label htmlFor="sv-nummer" className={labelClassName ?? "mb-1 block text-sm font-semibold text-ink"}>
-        SV-Nummer
-      </label>
-      <input
-        id="sv-nummer"
-        value={value}
-        inputMode="numeric"
-        placeholder={applies ? "1237 010180" : undefined}
-        onChange={(e) => onChange(e.target.value)}
-        onBlur={() => {
-          setTouched(true);
-          // Normalise to the conventional "NNNN TTMMJJ" spacing once the
-          // value is complete; anything else is left exactly as typed.
-          const formatted = formatSvnr(value);
-          if (formatted !== value) onChange(formatted);
-        }}
-        aria-invalid={showError || undefined}
-        aria-describedby={showError ? "sv-nummer-error" : undefined}
-        className={`w-full rounded border px-3 py-2 text-sm ${showError ? "border-danger-solid" : "border-border"}`}
-      />
-      {showError && (
-        <p id="sv-nummer-error" className="mt-1 text-xs font-semibold text-danger-text">
-          {svnrErrorMessage(error)}
-        </p>
-      )}
-      {applies && !showError && (
-        <p className="mt-1 text-xs text-ink-muted">10 Ziffern: laufende Nummer, Prüfziffer, Geburtsdatum (TTMMJJ).</p>
-      )}
-    </div>
+    <TextField
+      label="SV-Nummer"
+      dense={dense}
+      value={value}
+      onChange={onChange}
+      inputMode="numeric"
+      placeholder={applies ? "1237 010180" : undefined}
+      error={touched && error ? svnrErrorMessage(error) : null}
+      hint={applies ? "10 Ziffern: laufende Nummer, Prüfziffer, Geburtsdatum (TTMMJJ)." : undefined}
+      onBlur={() => {
+        setTouched(true);
+        // Normalise to the conventional "NNNN TTMMJJ" spacing once the value
+        // is complete; anything else is left exactly as typed.
+        const formatted = formatSvnr(value);
+        if (formatted !== value) onChange(formatted);
+      }}
+    />
   );
 }