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/orgchart/AsOfPicker.tsx

@@ -2,6 +2,8 @@
 
 import { CalendarClock } from "lucide-react";
 import { usePathname, useRouter, useSearchParams } from "next/navigation";
+import { Button } from "@/components/ui/Button";
+import { FILTER_SELECT_CLASS } from "@/components/ui/Field";
 import { fmtDate } from "@/lib/format";
 
 type AsOfPickerProps = {
@@ -44,12 +46,12 @@ export function AsOfPicker({ asOf, today, projectedCount, historyStartsAt }: AsO
           type="date"
           value={asOf}
           onChange={(e) => setAsOf(e.target.value || undefined)}
-          className="rounded border border-border px-3 py-2 text-sm"
+          className={FILTER_SELECT_CLASS}
         />
         {!isToday && (
-          <button type="button" onClick={() => setAsOf(undefined)} className="text-xs font-semibold text-brand-700 hover:underline">
+          <Button variant="ghost" size="sm" onClick={() => setAsOf(undefined)} className="!px-1 text-brand-700 hover:!bg-transparent hover:underline">
             Heute
-          </button>
+          </Button>
         )}
         <span className="text-xs text-ink-muted">
           {isToday ? "Aktuelle Organisationsstruktur." : `Struktur zum ${fmtDate(asOf)}.`}