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/bootstrap.php

@@ -12,6 +12,10 @@ if (session_status() !== PHP_SESSION_ACTIVE) {
     session_start();
 }
 
+if (!isset($_SESSION['validation_errors']) || !is_array($_SESSION['validation_errors'])) {
+    $_SESSION['validation_errors'] = [];
+}
+
 require_once __DIR__ . '/lib/_sql.php';
 $sql = new SQL();