ver. 0.300: Manifest-based update system with checksum verification and file backup

Replaces the manual ZIP packaging workflow with an automated build script.
UpdateRepository now supports both manifest JSON format (new) and legacy
_sql.txt/_files.txt format (fallback), enabling a smooth transition for
existing client instances.

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

updates/changelog.php

@@ -1,3 +1,7 @@
+<b>ver. 0.300 - 21.02.2026</b><br />
+- NEW - System aktualizacji oparty na manifestach JSON (checksum SHA256, backup plików, automatyczny build)
+- NEW - Panel logu aktualizacji w panelu admina
+<hr>
 <b>ver. 0.299 - 21.02.2026</b><br />
 - NEW - Ukrywanie/pokazywanie kolumn w tabelach admina (toggle switch + localStorage)
 <hr>

updates/versions.php

@@ -1,5 +1,5 @@
 <?
-$current_ver = 299;
+$current_ver = 300;
 
 for ($i = 1; $i <= $current_ver; $i++)
 {