Fix thanks and error page routing for both languages

- Add dedicated routes for /thanks, /error, /en/thanks, and /en/error
- Create missing error.en.txt content file
- Add title to error.fr.txt

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

content/error/error.en.txt

@@ -0,0 +1,5 @@
+Title: Error
+
+----
+
+Template: default

content/error/error.fr.txt

@@ -1 +1,5 @@
-Uuid: kcrqkszqasludg2h
+Title: Erreur
+
+----
+
+Uuid: kcrqkszqasludg2h

site/config/config.php

@@ -37,6 +37,44 @@ return [
                 return null;
             }
         ],
+        // English thanks page
+        [
+            'pattern' => 'en/thanks',
+            'action' => function() {
+                $thanks = page('thanks');
+                if ($thanks) {
+                    site()->visit($thanks, 'en');
+                    return $thanks;
+                }
+                return null;
+            }
+        ],
+        // English error page
+        [
+            'pattern' => 'en/error',
+            'action' => function() {
+                $error = page('error');
+                if ($error) {
+                    site()->visit($error, 'en');
+                    return $error;
+                }
+                return null;
+            }
+        ],
+        // French thanks page
+        [
+            'pattern' => 'thanks',
+            'action' => function() {
+                return page('thanks');
+            }
+        ],
+        // French error page
+        [
+            'pattern' => 'error',
+            'action' => function() {
+                return page('error');
+            }
+        ],
         // French products (default)
         [
             'pattern' => '(:any)',