Phase 3: Hire wizard, draft resume, and two real SQL bugfixes

- components/hire/: 4-step Hire Wizard (Person/Position/Vertrag/
  Zusammenfassung) matching sec4.4, with a HireWizardProvider context so it
  can be opened both from the global "+ Neueinstellung" button and from a
  "Fortsetzen" link on a saved draft.
- actions/hireDrafts.ts: save/delete hire_drafts (owner-scoped RLS already
  in place from Phase 1). Dashboard now shows the "Entwuerfe" card the
  Phase 1 plan deferred, since the wizard it depends on now exists.
- lib/positions.ts: shared open-positions loader (position number, org
  breadcrumb, resolved manager name) used by both the wizard and (later)
  the Positions page.

Two real bugs found via live testing and fixed in supabase/functions.sql:
1. hire_employee/rehire_employee: a two-branch CASE returning bare string
   literals defaults to `text`, not the target enum, so `status = case
   when ... then 'Geplant' else 'Aktiv' end` failed against the
   employment_status column. Fixed with an explicit ::employment_status
   cast on the whole CASE expression.
2. Postgres precedence gotcha: ->> and || sit at the *same* precedence
   tier and left-associate, so `payload->>'first_name' || ' ' ||
   payload->>'last_name'` does not group the way it reads - it tries to
   apply ->> to an intermediate text value and fails with "operator does
   not exist: text ->> unknown". Fixed by parenthesizing every ->>'...'
   expression that participates in a || chain.

Also fixed: hire_employee referenced v_position.title outside the branch
that assigns v_position, raising "record not assigned" whenever a hire
wasn't tied to a position_id; extracted a v_job_title variable instead.

Verified live end-to-end: wizard search -> select position -> submit
creates the employee, closes the position, and writes matching
employee_history + audit_log rows atomically.

components/shell/Topbar.tsx

@@ -23,16 +23,17 @@ function titleFor(pathname: string): string {
 type TopbarProps = {
   userLabel: string;
   role?: string;
+  canEdit: boolean;
 };
 
-export function Topbar({ userLabel, role }: TopbarProps) {
+export function Topbar({ userLabel, role, canEdit }: TopbarProps) {
   const pathname = usePathname();
 
   return (
     <header className="flex h-14 items-center justify-between border-b border-border bg-white px-6">
       <h1 className="text-base font-bold text-ink">{titleFor(pathname)}</h1>
       <div className="flex items-center gap-4">
-        <NewHireButton />
+        {canEdit && <NewHireButton />}
         <div className="flex items-center gap-2 border-l border-border pl-4 text-sm">
           <span className="font-semibold text-ink">{userLabel}</span>
           {role && <span className="text-xs text-ink-muted">({role === "hr_admin" ? "HR-Admin" : "Manager"})</span>}