feat(contact): server-side PHP mail handler for contact form

Fix #34: E-Mail-Versand via PHP

- PHP POST handler with server-side validation (name, email, message)
- mail() with From/Reply-To set to form email address
- Recipient: mki@kies-media.de
- Honeypot spam protection (hidden field)
- Minimum submit time check (3 seconds)
- Session-based rate limiting (60s between submissions)
- Header injection protection
- Error messages displayed above form
- Success message after successful send
- Form values preserved on validation errors
- Removed client-side mailto: JavaScript logic
- Added CSS for error display and honeypot hiding

css/haus-schleusingen.css

@@ -991,6 +991,36 @@ nav.scrolled .nav-hamburger span::after {
   transform: translateY(-1px);
 }
 
+.form-errors {
+  background: #fdf2f2;
+  border: 1px solid #e8a0a0;
+  padding: 1rem 1.2rem;
+  margin-bottom: 1.2rem;
+}
+
+.form-errors ul {
+  margin: 0;
+  padding: 0 0 0 1.2rem;
+  list-style: disc;
+}
+
+.form-errors li {
+  font-size: 0.85rem;
+  color: #9e2c2c;
+  line-height: 1.6;
+}
+
+.hp-field {
+  position: absolute;
+  left: -9999px;
+  top: -9999px;
+  width: 0;
+  height: 0;
+  overflow: hidden;
+  opacity: 0;
+  pointer-events: none;
+}
+
 .form-success {
   display: none;
   text-align: center;