feat: billableaua.de — add billable hours calculator

Interactive calculator: enter annual hours target + vacation days,
shows actual billable hours per working day after subtracting weekends,
vacation, and public holidays. Makes the absurdity of 2400h targets visible.

sites/billableaua.de/index.html

@@ -157,6 +157,91 @@
       font-weight: 300;
     }
 
+    /* Rechner */
+    .rechner {
+      padding: 120px 0;
+    }
+
+    .rechner h2 {
+      font-size: clamp(2.5rem, 7vw, 4rem);
+      font-weight: 900;
+      letter-spacing: -0.02em;
+      text-transform: uppercase;
+      margin-bottom: 48px;
+    }
+
+    .rechner h2 .dot { color: var(--red); }
+
+    .rechner-grid {
+      display: flex;
+      gap: 32px;
+      margin-bottom: 48px;
+    }
+
+    .rechner-input {
+      flex: 1;
+    }
+
+    .rechner-input label {
+      display: block;
+      font-size: 0.9rem;
+      color: var(--text-dim);
+      font-weight: 300;
+      letter-spacing: 0.08em;
+      text-transform: uppercase;
+      margin-bottom: 8px;
+    }
+
+    .rechner-input input {
+      width: 100%;
+      background: transparent;
+      border: 1px solid var(--text-muted);
+      color: var(--text);
+      font-family: inherit;
+      font-size: 2.4rem;
+      font-weight: 700;
+      padding: 12px 16px;
+      text-align: center;
+      outline: none;
+      transition: border-color 0.3s;
+      -moz-appearance: textfield;
+    }
+
+    .rechner-input input::-webkit-inner-spin-button,
+    .rechner-input input::-webkit-outer-spin-button {
+      -webkit-appearance: none;
+    }
+
+    .rechner-input input:focus {
+      border-color: var(--red);
+    }
+
+    .rechner-result {
+      font-size: clamp(1.3rem, 3vw, 1.8rem);
+      font-weight: 300;
+      color: var(--text-dim);
+      line-height: 2;
+    }
+
+    .rechner-result .num {
+      color: var(--red);
+      font-weight: 700;
+      font-size: 1.15em;
+    }
+
+    .rechner-result .verdict {
+      display: block;
+      margin-top: 24px;
+      font-size: clamp(1.5rem, 4vw, 2.2rem);
+      font-weight: 600;
+      color: var(--text);
+    }
+
+    @media (max-width: 640px) {
+      .rechner { padding: 80px 0; }
+      .rechner-grid { flex-direction: column; gap: 24px; }
+    }
+
     /* Silence */
     .silence {
       padding: 160px 0;
@@ -227,6 +312,25 @@
 
   <div class="line"></div>
 
+  <section class="rechner">
+    <div class="wrap">
+      <h2 class="reveal">Rechne selbst<span class="dot">.</span></h2>
+      <div class="rechner-grid reveal">
+        <div class="rechner-input">
+          <label>Billable-Stunden / Jahr</label>
+          <input type="number" id="hours" value="2400" min="0" max="5000">
+        </div>
+        <div class="rechner-input">
+          <label>Urlaubstage</label>
+          <input type="number" id="vacation" value="25" min="0" max="60">
+        </div>
+      </div>
+      <div class="rechner-result reveal" id="result"></div>
+    </div>
+  </section>
+
+  <div class="line"></div>
+
   <section class="kritik">
     <div class="wrap">
 
@@ -277,6 +381,37 @@
     }, { threshold: 0.15 });
 
     document.querySelectorAll('.reveal').forEach(el => observer.observe(el));
+
+    // Rechner
+    function calculate() {
+      const hours = parseInt(document.getElementById('hours').value) || 0;
+      const vacation = parseInt(document.getElementById('vacation').value) || 0;
+      const weekends = 104;
+      const feiertage = 10;
+      const workingDays = 365 - weekends - vacation - feiertage;
+      const perDay = workingDays > 0 ? (hours / workingDays) : 0;
+      const perDayStr = perDay.toFixed(1).replace('.', ',');
+
+      let verdict = '';
+      if (perDay > 10) {
+        verdict = '<span class="verdict">Das ist kein Job. <span class="dim">Das ist ein Lifestyle.</span></span>';
+      } else if (perDay > 8) {
+        verdict = '<span class="verdict">Jeden Tag &uuml;ber <span class="num">8</span> Stunden billable. <span class="dim">Ohne Mittagspause, ohne Admin, ohne Atmen.</span></span>';
+      } else if (perDay > 6) {
+        verdict = '<span class="verdict">Klingt machbar? <span class="dim">Vergiss nicht: billable &ne; Arbeitszeit.</span></span>';
+      }
+
+      document.getElementById('result').innerHTML =
+        '<span class="num">' + workingDays + '</span> Arbeitstage ' +
+        '<span class="dim">(365 &minus; ' + weekends + ' Wochenenden &minus; ' + vacation + ' Urlaub &minus; ' + feiertage + ' Feiertage)</span><br>' +
+        '<span class="num">' + hours.toLocaleString('de-DE') + '</span> Stunden &divide; <span class="num">' + workingDays + '</span> Tage = ' +
+        '<span class="num">' + perDayStr + '</span> Stunden pro Tag.' +
+        verdict;
+    }
+
+    document.getElementById('hours').addEventListener('input', calculate);
+    document.getElementById('vacation').addEventListener('input', calculate);
+    calculate();
   </script>
 
 </body>