fix: company save 500 error — catch DB exceptions, show error to user

- CompanyRepository::save() now catches PDOException and throws a
  RuntimeException with a human-readable message (duplicate name, etc.)
- Also fixes unchecked checkboxes (requires_approval, is_active) not
  being saved as 0 on UPDATE
- index.php save handler catches RuntimeException and redirects back to
  the form with an ?error= param instead of crashing with 500
- companyEdit() renders the error banner when ?error= is present

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin/DashboardController.php

@@ -1017,7 +1017,12 @@ HTML;
             $botOptions .= "<option value=\"{$val}\" {$sel}>{$lbl}</option>\n";
         }
 
-        $pageTitle = $isEdit ? 'Editar Empresa' : 'Nueva Empresa';
+        $pageTitle  = $isEdit ? 'Editar Empresa' : 'Nueva Empresa';
+        $errorBanner = '';
+        if (!empty($_GET['error'])) {
+            $errText = self::h(urldecode($_GET['error']));
+            $errorBanner = "<div style='background:#fef2f2;border:1px solid #fca5a5;color:#991b1b;padding:12px 16px;border-radius:8px;margin-bottom:16px;font-size:14px'>⚠ {$errText}</div>";
+        }
         echo Layout::open($pageTitle, 'companies', $user['name'] ?? 'Admin');
 
         // ── Tab: Números WhatsApp ─────────────────────────────────────────────
@@ -1251,6 +1256,7 @@ HTML : '';
     <h1 style="font-size:18px;font-weight:700;color:#111827">{$pageTitle}</h1>
     <a href="/admin/companies" class="btn-secondary" style="font-size:12px">← Empresas</a>
   </div>
+  {$errorBanner}
   {$tabsHtml}
 
   <!-- Tab General -->