dynamically cap down_pct slider based on start_capital

ui.js

@@ -280,6 +280,7 @@ const update_results = (result) => {
 // ===== Main recalc =====
 
 const recalc = () => {
+  clamp_down_pct();
   const p = get_params();
   const result = run_simulation(p);
 
@@ -289,6 +290,26 @@ const recalc = () => {
   build_table(result);
 };
 
+// ===== Down payment cap =====
+
+const clamp_down_pct = () => {
+  const home_price    = +document.getElementById('home_price').value;
+  const start_capital = +document.getElementById('start_capital').value;
+  const closing_pct   = +document.getElementById('closing_buy_pct').value;
+  const financed      = document.getElementById('finance_nebenkosten').checked;
+
+  const closing_costs = home_price * (closing_pct / 100);
+  const available     = financed ? start_capital : Math.max(0, start_capital - closing_costs);
+  const max_pct       = home_price > 0 ? Math.min(100, (available / home_price) * 100) : 100;
+
+  const slider = document.getElementById('down_pct_slider');
+  const num    = document.getElementById('down_pct');
+  if (+num.value > max_pct) {
+    num.value    = Math.round(max_pct * 10) / 10;
+    slider.value = num.value;
+  }
+};
+
 // ===== Slider ↔ number sync =====
 
 const wire_slider = (base_id) => {
@@ -317,9 +338,13 @@ const SLIDER_IDS = [
 SLIDER_IDS.forEach(wire_slider);
 
 // wire plain number inputs (no slider)
-document.getElementById('finance_nebenkosten').addEventListener('change', recalc);
+document.getElementById('finance_nebenkosten').addEventListener('change', () => { clamp_down_pct(); recalc(); });
 
-['home_price', 'insurance', 'monthly_rent', 'renters_ins', 'start_capital'].forEach((id) => {
+['home_price', 'start_capital'].forEach((id) => {
+  document.getElementById(id).addEventListener('input', () => { clamp_down_pct(); recalc(); });
+});
+
+['insurance', 'monthly_rent', 'renters_ins'].forEach((id) => {
   document.getElementById(id).addEventListener('input', recalc);
 });