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>

public/index.php

@@ -241,12 +241,17 @@ $routes = [
     // ─── Guardar empresa (crear/actualizar) ─────────────────────────────────
     ['POST', '/admin/company/save', fn() => (function () {
         SessionAuth::require();
-        $data = $_POST;
-        $id   = CompanyRepository::save($data);
-        if ($id > 0) {
-            header('Location: /admin/companies?msg=' . ($data['id'] ?? 0 > 0 ? 'updated' : 'created'));
-        } else {
-            header('Location: /admin/companies?msg=error');
+        $data  = $_POST;
+        $isNew = empty($data['id']) || (int)$data['id'] === 0;
+        try {
+            CompanyRepository::save($data);
+            header('Location: /admin/companies?msg=' . ($isNew ? 'created' : 'updated'));
+        } catch (\RuntimeException $e) {
+            $errMsg   = urlencode($e->getMessage());
+            $redirect = $isNew
+                ? '/admin/companies/new?error=' . $errMsg
+                : '/admin/company/edit?id=' . (int)$data['id'] . '&error=' . $errMsg;
+            header('Location: ' . $redirect);
         }
         exit;
     })()],