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/EmployeeTree.tsx

@@ -1,9 +1,11 @@
 "use client";
 
-import { ChevronDown, ChevronRight, Search } from "lucide-react";
+import { ChevronDown, ChevronRight } from "lucide-react";
 import Link from "next/link";
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { Avatar } from "@/components/ui/Avatar";
+import { Button } from "@/components/ui/Button";
+import { SearchInput } from "@/components/ui/SearchInput";
 import { SegmentedControl } from "@/components/ui/SegmentedControl";
 import { LazyGraphOrgChart } from "./LazyGraphOrgChart";
 import type { ChartNode, OrgEmployee } from "./types";
@@ -167,29 +169,13 @@ export function EmployeeTree({ employees, focusId = null }: { employees: OrgEmpl
   return (
     <div className="rounded border border-border bg-white p-4">
       <div className="mb-4 flex flex-wrap items-center gap-3">
-        <div className="flex min-w-[240px] flex-1 items-center gap-2 rounded border border-border px-3 py-2">
-          <Search className="h-4 w-4 shrink-0 text-ink-muted" />
-          <input
-            value={query}
-            onChange={(e) => setQuery(e.target.value)}
-            placeholder="Name, Pers.-Nr., Titel…"
-            className="w-full text-sm text-ink outline-none placeholder:text-ink-muted"
-          />
-        </div>
-        <button
-          type="button"
-          onClick={() => setExpanded(new Set(root.map((r) => r.id)))}
-          className="rounded border border-border px-3 py-1.5 text-sm font-semibold text-ink-body hover:bg-surface"
-        >
+        <SearchInput label="Organigramm durchsuchen" placeholder="Name, Pers.-Nr., Titel…" value={query} onChange={setQuery} />
+        <Button variant="secondary" size="sm" onClick={() => setExpanded(new Set(root.map((r) => r.id)))}>
           Bereiche anzeigen
-        </button>
-        <button
-          type="button"
-          onClick={() => setExpanded(new Set())}
-          className="rounded border border-border px-3 py-1.5 text-sm font-semibold text-ink-body hover:bg-surface"
-        >
+        </Button>
+        <Button variant="secondary" size="sm" onClick={() => setExpanded(new Set())}>
           Alles einklappen
-        </button>
+        </Button>
         <SegmentedControl<ViewMode> value={mode} onChange={setMode} options={[{ value: "list", label: "Liste" }, { value: "graph", label: "Grafisch" }]} />
       </div>
       {mode === "list" ? (