Fix HTTP 500 on PHP < 8.2: drop standalone true union type

registerUser() declared `: string|true`; the bare `true` type requires
PHP 8.2, so including auth.php was a fatal error on PHP 8.0/8.1 — every
login/API call returned 500 while install.php (which doesn't include
auth.php) worked. Use string|bool instead. Also install a JSON exception
handler so future server errors surface a message to the SPA instead of
a blank 500.

includes/db.php

@@ -1,6 +1,19 @@
 <?php
 require_once __DIR__ . '/../config.php';
 
+// Uncaught exceptions in web requests become JSON errors (the SPA surfaces
+// them) instead of blank 500 pages; details still go to the PHP error log.
+if (PHP_SAPI !== 'cli') {
+    set_exception_handler(function (Throwable $e): void {
+        error_log($e);
+        http_response_code(500);
+        if (!headers_sent()) {
+            header('Content-Type: application/json');
+        }
+        echo json_encode(['error' => 'Server error: ' . $e->getMessage()]);
+    });
+}
+
 function db(): PDO
 {
     static $pdo = null;