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/NewHireButton.tsx

@@ -1,16 +1,15 @@
 "use client";
 
 import { Plus } from "lucide-react";
-import { useToast } from "@/components/ui/Toast";
+import { useHireWizard } from "@/components/hire/HireWizardContext";
 
-// Stub for Phase 1: the real 4-step hire wizard (§4.4) ships in a later phase.
 export function NewHireButton() {
-  const { showToast } = useToast();
+  const { openWizard } = useHireWizard();
 
   return (
     <button
       type="button"
-      onClick={() => showToast("Der Neueinstellungs-Assistent folgt in einer späteren Ausbaustufe.", "info")}
+      onClick={() => openWizard()}
       className="flex items-center gap-1.5 rounded bg-brand-500 px-3 py-1.5 text-sm font-semibold text-white hover:bg-brand-600"
     >
       <Plus className="h-4 w-4" />