Show positions that do not exist yet, and let them be corrected

Two gaps in the positions view, both reported from use.

A position dated into the future was invisible. loadOpenPositions required
valid_from <= today, so a position decided now and effective at the quarter
boundary appeared nowhere until the day it began. The database already held
one — 60000824 "Neue Position", effective 01.09. — created through the
application and shown on no screen since.

Future positions now have their own section rather than joining the vacancy
list. They are a different statement: "nobody is here" and "this does not
exist yet" should not be counted together, and a position starting 01.10.
read as a vacancy nobody was filling.

Positions could only be created and deleted. Fixing a typo in the job title
meant deleting and recreating — with a new position number, which appears in
job postings, budgets and audit entries, and whose trail then breaks.
update_position keeps the number and records old and new values per field,
using the audit detail added earlier today.

Three things it refuses, as guards rather than remarks:

  - Moving an occupied position to another unit. That is a transfer, with
    history and reporting line, and belongs to the person — otherwise
    someone changes department silently.
  - Ending an occupied position, which would leave an assignment without
    one.
  - A second chief position in a unit, or an end before the start.

Verified against the live database, all rolled back: each guard fires with
its own message, the permitted edits go through, the audit entry carries the
changed fields. Open positions stay at 9 and the future one now appears in
its own section.

ESLint caught me priming the dialog's fields from an effect. Replaced by a
key on the component, so React rebuilds it per position and the fields
initialise from props — which also removes the flash of the previous
position's values on second open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

components/positions/EditPositionModal.tsx

@@ -0,0 +1,138 @@
+"use client";
+
+import { useRouter } from "next/navigation";
+import { useState } from "react";
+import { updatePosition } from "@/actions/positions";
+import { Button } from "@/components/ui/Button";
+import { SelectField, TextField } from "@/components/ui/Field";
+import { Modal } from "@/components/ui/Modal";
+import { useToast } from "@/components/ui/Toast";
+import type { OpenPositionResolved } from "@/lib/positions";
+import type { UnitOption } from "./CreatePositionModal";
+
+// Ändern statt löschen und neu anlegen.
+//
+// Die Planstellennummer steht in Ausschreibungen, Budgets und
+// Protokolleinträgen. Wer wegen eines Tippfehlers in der Tätigkeit eine neue
+// Nummer vergibt, macht die alten Bezüge wertlos — deshalb gibt es diesen
+// Dialog.
+
+/**
+ * Erwartet eine Planstelle, keine „vielleicht keine".
+ *
+ * Die aufrufende Seite hängt einen `key` mit der Kennung daran und rendert
+ * ihn nur, solange etwas bearbeitet wird. Dadurch baut React den Dialog je
+ * Planstelle neu auf, und die Felder lassen sich direkt aus den Eigenschaften
+ * vorbelegen — statt sie in einem Effekt nachzuziehen, der beim zweiten
+ * Öffnen kurz die Werte der vorigen Stelle zeigt.
+ */
+export function EditPositionModal({
+  position,
+  units,
+  onClose,
+}: {
+  position: OpenPositionResolved;
+  units: UnitOption[];
+  onClose: () => void;
+}) {
+  const { showToast } = useToast();
+  const router = useRouter();
+  const [jobTitle, setJobTitle] = useState(position.title);
+  const [orgUnitId, setOrgUnitId] = useState(position.org_unit_id);
+  const [isChief, setIsChief] = useState(position.is_chief);
+  const [validFrom, setValidFrom] = useState(position.valid_from);
+  const [validTo, setValidTo] = useState(position.valid_to ?? "");
+  const [pending, setPending] = useState(false);
+
+  const unit = units.find((u) => u.id === orgUnitId);
+  // Die eigene Leitung zählt nicht als Hindernis für sich selbst.
+  const chiefTaken = Boolean(unit?.hasChief) && !(position.is_chief && orgUnitId === position.org_unit_id);
+
+  async function handleSubmit() {
+    if (!jobTitle.trim() || !orgUnitId || !validFrom) {
+      showToast("Tätigkeit, Einheit und Gültigkeitsbeginn sind Pflicht.", "error");
+      return;
+    }
+    setPending(true);
+    const result = await updatePosition({
+      position_id: position.id,
+      org_unit_id: orgUnitId,
+      job_title: jobTitle.trim(),
+      is_chief: isChief && !chiefTaken,
+      valid_from: validFrom,
+      valid_to: validTo || null,
+    });
+    setPending(false);
+    if (result.success) {
+      showToast("Planstelle geändert.");
+      router.refresh();
+      onClose();
+    } else {
+      // Die Meldungen der Datenbankfunktion sind für die Oberfläche
+      // geschrieben („Diese Planstelle ist vergeben …“) und werden gezeigt.
+      showToast(result.error ?? "Die Änderung war nicht möglich.", "error");
+    }
+  }
+
+  return (
+    <Modal
+      open
+      onClose={onClose}
+      title={`Planstelle ${position.position_number}`}
+      footer={
+        <>
+          <Button variant="secondary" onClick={onClose}>
+            Abbrechen
+          </Button>
+          <Button onClick={handleSubmit} pending={pending}>
+            Speichern
+          </Button>
+        </>
+      }
+    >
+      <div className="flex flex-col gap-3">
+        <TextField label="Tätigkeit" required value={jobTitle} onChange={setJobTitle} />
+
+        <SelectField
+          label="Organisationseinheit"
+          required
+          value={orgUnitId}
+          onChange={setOrgUnitId}
+          placeholder="Bitte wählen…"
+          options={units.map((u) => ({
+            value: u.id,
+            label: `${"  ".repeat(u.depth)}${u.name} (${u.unit_type})`,
+          }))}
+        />
+
+        <div className="grid grid-cols-1 gap-3 sm:grid-cols-2">
+          <TextField label="Gültig ab" required type="date" value={validFrom} onChange={setValidFrom} />
+          <TextField label="Gültig bis" type="date" value={validTo} onChange={setValidTo} />
+        </div>
+
+        <label className="flex items-start gap-2 text-sm text-ink-body">
+          <input
+            type="checkbox"
+            className="mt-0.5 h-4 w-4 rounded border-border text-brand-600 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-500"
+            checked={isChief && !chiefTaken}
+            disabled={chiefTaken}
+            onChange={(e) => setIsChief(e.target.checked)}
+          />
+          <span>
+            Leitungsplanstelle
+            {chiefTaken && (
+              <span className="block text-xs text-ink-muted">
+                Für {unit?.name} besteht bereits eine Leitungsplanstelle.
+              </span>
+            )}
+          </span>
+        </label>
+
+        <p className="rounded-md bg-surface px-3 py-2 text-xs leading-relaxed text-ink-muted">
+          Die Planstellennummer bleibt. Ist die Stelle vergeben, lassen sich Einheit und Gültigkeitsende nicht ändern —
+          ein Wechsel der Einheit ist eine Versetzung und gehört zur Person, nicht zur Stelle.
+        </p>
+      </div>
+    </Modal>
+  );
+}