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/floors/save.php

@@ -37,6 +37,7 @@ if ($buildingId <= 0) {
 // Falls Fehler: zurück zum Edit-Formular
 if (!empty($errors)) {
     $_SESSION['error'] = implode(', ', $errors);
+    $_SESSION['validation_errors'] = $errors;
     $redirectUrl = $floorId ? "?module=floors&action=edit&id=$floorId" : "?module=floors&action=edit";
     header("Location: $redirectUrl");
     exit;
@@ -50,6 +51,7 @@ if ($floorSvgContent !== '') {
     $storedSvgPath = storeSvgEditorContent($sql, $floorId, $floorSvgContent);
     if ($storedSvgPath === false) {
         $_SESSION['error'] = "SVG aus dem Editor konnte nicht gespeichert werden";
+        $_SESSION['validation_errors'] = ["SVG aus dem Editor konnte nicht gespeichert werden"];
         $redirectUrl = $floorId ? "?module=floors&action=edit&id=$floorId" : "?module=floors&action=edit";
         header("Location: $redirectUrl");
         exit;