add Startkapital input for renter initial savings

calc.js

@@ -30,7 +30,7 @@ function run_simulation(p) {
   // --- state ---
   let balance = loan;
   let home_value = p.home_price;
-  let portfolio = upfront_buy; // renter invests this lump sum
+  let portfolio = upfront_buy + (p.start_capital || 0); // renter invests this lump sum + any extra savings
   let monthly_rent = p.monthly_rent;
 
   let cum_buy_cost = upfront_buy; // running cash spent on buying

index.html

@@ -171,6 +171,14 @@
       <section class="card inputs-rent">
         <h2 class="col-header rent-header">Mieten</h2>
 
+        <div class="input-group">
+          <label for="start_capital">Startkapital <span class="hint">(zusätzl. Ersparnisse)</span></label>
+          <div class="num-unit euro-input">
+            <span class="unit prefix">€</span>
+            <input type="number" id="start_capital" min="0" max="10000000" step="1000" value="0" />
+          </div>
+        </div>
+
         <div class="input-group">
           <label for="monthly_rent">Kaltmiete (monatlich)</label>
           <div class="num-unit euro-input">

ui.js

@@ -24,6 +24,7 @@ const get_params = () => ({
   monthly_rent:    +document.getElementById('monthly_rent').value,
   rent_increase:   +document.getElementById('rent_increase').value,
   renters_ins:     +document.getElementById('renters_ins').value,
+  start_capital:   +document.getElementById('start_capital').value,
 });
 
 // ===== Charts =====
@@ -227,7 +228,7 @@ const SLIDER_IDS = [
 SLIDER_IDS.forEach(wire_slider);
 
 // wire plain number inputs (no slider)
-['home_price', 'insurance', 'monthly_rent', 'renters_ins'].forEach((id) => {
+['home_price', 'insurance', 'monthly_rent', 'renters_ins', 'start_capital'].forEach((id) => {
   document.getElementById(id).addEventListener('input', recalc);
 });