feat: implement package 1 session and validation feedback

- add session validation_errors bootstrap initialization

- render global flash + validation messages in header

- remove footer alert-based flash handling

- persist structured validation errors across save handlers

- mark NEXT_STEPS package 1 tasks as done

app/modules/connections/save.php

@@ -88,6 +88,7 @@ if ($isEndpointUsed($portBType, $portBId)) {
 
 if (!empty($errors)) {
     $_SESSION['error'] = implode(', ', $errors);
+    $_SESSION['validation_errors'] = $errors;
     $redirectUrl = $connId ? "?module=connections&action=edit&id=$connId" : "?module=connections&action=edit";
     header("Location: $redirectUrl");
     exit;
@@ -123,6 +124,7 @@ if ($connId > 0) {
 
     if ($connectionTypeId <= 0) {
         $_SESSION['error'] = "Kein Verbindungstyp verfuegbar";
+        $_SESSION['validation_errors'] = ["Kein Verbindungstyp verfuegbar"];
         header("Location: ?module=connections&action=edit");
         exit;
     }