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

js/haus-schleusingen.js

@@ -71,35 +71,7 @@ $(function () {
     }
   });
 
-  // Form submit – opens email client with pre-filled mailto: link
-  $("#contactForm").on("submit", function (e) {
-    e.preventDefault();
-
-    var fname = $("#fname").val().trim();
-    var lname = $("#lname").val().trim();
-    var email = $("#email").val().trim();
-    var phone = $("#phone").val().trim();
-    var interest = $("#interest").val();
-    var message = $("#message").val().trim();
-
-    var subject = "Kontaktanfrage: " + interest;
-    var body = "Von: " + fname + " " + lname + "\n";
-    body += "E-Mail: " + email + "\n";
-    if (phone) body += "Telefon: " + phone + "\n";
-    body += "Anliegen: " + interest + "\n\n";
-    body += message;
-
-    var mailto =
-      "mailto:mki@kies-media.de" +
-      "?subject=" + encodeURIComponent(subject) +
-      "&body=" + encodeURIComponent(body);
-
-    window.location.href = mailto;
-
-    // Show success message
-    $("#contactForm").hide();
-    $("#formSuccess").fadeIn(400);
-  });
+  // Form submit is handled server-side by PHP – no JS intervention needed.
 });
 
 // Mobile hamburger menu (vanilla JS)