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/rooms/save.php

@@ -18,6 +18,15 @@ $comment = trim((string)($_POST['comment'] ?? ''));
 $rawPolygon = trim((string)($_POST['polygon_points'] ?? ''));
 
 if ($name === '' || $floorId <= 0) {
+    $errors = [];
+    if ($name === '') {
+        $errors[] = 'Name ist erforderlich';
+    }
+    if ($floorId <= 0) {
+        $errors[] = 'Stockwerk ist erforderlich';
+    }
+    $_SESSION['error'] = implode(', ', $errors);
+    $_SESSION['validation_errors'] = $errors;
     $redirect = $roomId > 0 ? "?module=rooms&action=edit&id=$roomId" : "?module=rooms&action=edit&floor_id=$floorId";
     header("Location: $redirect");
     exit;
@@ -100,6 +109,7 @@ if (roomsHasPolygonColumn($sql)) {
     }
 }
 
+$_SESSION['success'] = $roomId > 0 ? 'Raum gespeichert' : 'Raum erstellt';
 header('Location: ?module=locations&action=list');
 exit;