fix: timestamp handling for database backup in Web UI (#27359)

* Fix #26502: Fix timestamp handling for database backup in Web UI

Frontend parsed backup timestamps as UTC, but they were in the
server's local timezone, causing wrong relative times.

Add `timezone` field to DatabaseBackupDto to expose server timezone.
Update frontend to parse timestamps using this timezone.
Convert timestamps to user's local timezone before rendering.
Fallback to browser timezone if server timezone is missing.

Ensures correct relative time display in Web UI.

* fix: regenerate open-api types and remove custom backup type

- Ran `make open-api` to update types based on backend changes
- Removed custom BackupWithTimezone type in MaintenanceBackupsList
- Updated timezone props to use the newly generated native type

* fix: simplify timezone handling for database backups

- Updated DatabaseBackupDto to make timezone a required property
- Removed manual DateTime.local().zoneName fallbacks
- Cleaned up type casts after regenerating OpenAPI types

* fix: Add missing newline at end of spec file

mobile/openapi/lib/model/database_backup_dto.dart

@@ -15,30 +15,36 @@ class DatabaseBackupDto {
   DatabaseBackupDto({
     required this.filename,
     required this.filesize,
+    required this.timezone,
   });
 
   String filename;
 
   num filesize;
 
+  String timezone;
+
   @override
   bool operator ==(Object other) => identical(this, other) || other is DatabaseBackupDto &&
     other.filename == filename &&
-    other.filesize == filesize;
+    other.filesize == filesize &&
+    other.timezone == timezone;
 
   @override
   int get hashCode =>
     // ignore: unnecessary_parenthesis
     (filename.hashCode) +
-    (filesize.hashCode);
+    (filesize.hashCode) +
+    (timezone.hashCode);
 
   @override
-  String toString() => 'DatabaseBackupDto[filename=$filename, filesize=$filesize]';
+  String toString() => 'DatabaseBackupDto[filename=$filename, filesize=$filesize, timezone=$timezone]';
 
   Map<String, dynamic> toJson() {
     final json = <String, dynamic>{};
       json[r'filename'] = this.filename;
       json[r'filesize'] = this.filesize;
+      json[r'timezone'] = this.timezone;
     return json;
   }
 
@@ -53,6 +59,7 @@ class DatabaseBackupDto {
       return DatabaseBackupDto(
         filename: mapValueOfType<String>(json, r'filename')!,
         filesize: num.parse('${json[r'filesize']}'),
+        timezone: mapValueOfType<String>(json, r'timezone')!,
       );
     }
     return null;
@@ -102,6 +109,7 @@ class DatabaseBackupDto {
   static const requiredKeys = <String>{
     'filename',
     'filesize',
+    'timezone',
   };
 }