fix: UTF-8 BOM in update SQL files causing MariaDB syntax error

PowerShell 5.1 Out-File -Encoding UTF8 adds BOM (EF BB BF) which
breaks SQL execution on production. Also fix manifest JSON serializing
full PS objects instead of plain strings.

- build-update.ps1: use UTF8Encoding($false) for all file writes
- build-update.ps1: force .ToString() on Get-Content results
- UpdateRepository.php: strip BOM and normalize line endings in executeSql
- Rebuild ver_0.304 package files (clean SQL + manifest)

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

updates/0.30/ver_0.304_sql.txt

@@ -0,0 +1,2 @@
+ALTER TABLE pp_shop_payment_methods ADD COLUMN min_order_amount DECIMAL(10,2) DEFAULT NULL;
+ALTER TABLE pp_shop_payment_methods ADD COLUMN max_order_amount DECIMAL(10,2) DEFAULT NULL;