Fase 2: read-only user role with per-category view toggles

Adds a third account type 'user' alongside super/admin: no create/edit/delete
rights on qr codes, view access to dynamic/static lists gated per-account by
two admin-controlled toggles (can_view_static, can_view_dynamic), and always
full visibility into the dashboard/reports regardless of those toggles.

- New columns can_view_static/can_view_dynamic on users (migrations/003)
- Users class + form_users.php: 'user' type option with the two toggles
- Access control: dynamic_qrcode.php/static_qrcode.php/bulk_action.php reject
  all mutations for type=user; dynamic_qrcodes.php/static_qrcodes.php enforce
  the view toggle and show all codes (no owner scoping, since 'user' owns none)
- Sidebar and list tables hide add/edit/delete/bulk UI for the read-only role
- index.php dashboard stats are unscoped for both 'super' and 'user'

src/dynamic_qrcode.php

@@ -3,6 +3,12 @@ require_once 'includes/bootstrap.php';
 require_once BASE_PATH.'/includes/auth_validate.php';
 require_once BASE_PATH . '/lib/DynamicQrcode/DynamicQrcode.php';
 
+if ($_SESSION['type'] === 'user') {
+    $_SESSION['failure'] = 'The "user" role is read-only and cannot create, edit or delete qr codes.';
+    header('Location: index.php');
+    exit;
+}
+
 $dynamic_qrcode_instance = new DynamicQrcode();
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {