Merge all text_constants into a single canonical file

Four separate text_constants.inc.php files (root, mysyde/common,
mysyde/common/classes, mysyde/admin) contained overlapping and
diverging constants. Merged all 1596 unique keys—zero value conflicts—
into mysyde/admin/text_constants.inc.php, which Translate.php already
loads. The three other files are now thin stubs that require_once the
admin file, so all existing include paths continue to work.

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

text_constants.inc.php

@@ -0,0 +1,3 @@
+<?php
+// Merged into mysyde/admin/text_constants.inc.php
+require_once __DIR__ . '/mysyde/admin/text_constants.inc.php';