feat: Payload-Preview im Chat-Menü + Reminder von hinten gezählt

- Neuer Endpoint GET /api/sessions/{id}/payload-preview
  Gibt den exakten Anthropic-API-Payload als JSON zurück (read-only)
- Chat-Menü: 'Payload anzeigen' öffnet Modal mit formatiertem JSON
- Reminder-Logik umgebaut: halbes Intervall (15) vor Ende,
  dann volle Intervalle (30) rückwärts statt von vorne gezählt
- CSS: Payload-Modal Styling, kleine Variable-Fixes

wwwroot/js/modals.js

@@ -76,6 +76,26 @@ async function showSystemPromptModal(sessionId) {
   }
 }
 
+// ── Payload Preview Modal ──
+
+async function showPayloadPreviewModal(sessionId) {
+  const { body } = _createModal('Payload Preview', { modal: 'payload-preview-modal' });
+  body.innerHTML = '<div class="sp-loading">Laden...</div>';
+
+  try {
+    var planMode = typeof _planModeActive !== 'undefined' && _planModeActive;
+    var payload = await apiGetPayloadPreview(sessionId, planMode);
+    body.innerHTML = '';
+
+    var pre = document.createElement('pre');
+    pre.className = 'payload-json';
+    pre.textContent = JSON.stringify(payload, null, 2);
+    body.appendChild(pre);
+  } catch (e) {
+    body.innerHTML = '<div class="sp-loading text-danger">Fehler beim Laden des Payloads</div>';
+  }
+}
+
 // ── Command Output Modal ──
 
 function showCommandOutputModal(name, command, result) {