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/positions/PositionsPageClient.tsx

@@ -4,6 +4,7 @@ import { Plus, Trash2 } from "lucide-react";
 import { useRouter } from "next/navigation";
 import { useState } from "react";
 import { deletePosition } from "@/actions/positions";
+import { Button } from "@/components/ui/Button";
 import { useToast } from "@/components/ui/Toast";
 import { fmtDate, todayIso } from "@/lib/format";
 import type { OpenPositionResolved } from "@/lib/positions";
@@ -40,14 +41,10 @@ export function PositionsPageClient({ openPositions, teams }: PositionsPageClien
       <div className="rounded border border-border bg-white p-4">
         <div className="mb-3 flex flex-wrap items-center justify-between gap-2">
           <h2 className="text-sm font-bold text-ink">Offene Positionen ({openPositions.length})</h2>
-          <button
-            type="button"
-            onClick={() => setCreateOpen(true)}
-            className="flex items-center gap-1.5 rounded bg-brand-500 px-3 py-2 text-sm font-semibold text-white hover:bg-brand-600"
-          >
+          <Button onClick={() => setCreateOpen(true)}>
             <Plus className="h-4 w-4" />
             Position ausschreiben
-          </button>
+          </Button>
         </div>
         {openPositions.length === 0 ? (
           <p className="text-sm text-ink-muted">Derzeit keine offenen Positionen.</p>
@@ -59,15 +56,15 @@ export function PositionsPageClient({ openPositions, teams }: PositionsPageClien
                 <div key={p.id} className="flex flex-col rounded border border-border p-3">
                   <div className="flex items-start justify-between gap-2">
                     <div className="text-sm font-semibold text-ink">{p.title}</div>
-                    <button
-                      type="button"
+                    <Button
+                      variant="icon"
                       onClick={() => handleDelete(p.id)}
-                      disabled={deletingId === p.id}
-                      aria-label="Position löschen"
-                      className="-mr-1 -mt-1 rounded p-2 text-ink-muted hover:text-danger-solid disabled:opacity-50"
+                      pending={deletingId === p.id}
+                      aria-label={`Position ${p.title} löschen`}
+                      className="-mr-1 -mt-1 hover:!text-danger-solid"
                     >
                       <Trash2 className="h-3.5 w-3.5" />
-                    </button>
+                    </Button>
                   </div>
                   <div className="text-xs text-ink-muted">
                     {p.position_number} · {p.orgLabel}