fix: prevent special character corruption by enforcing UTF-8 in escaping

[anonymoususer72041]

Summary

This PR reduces special character and non-ASCII character rendering issues across the UI by ensuring OpenCATS consistently uses an explicit UTF-8 encoding when escaping content.

It updates several htmlspecialchars() call sites to pass HTML_ENCODING or AJAX_ENCODING instead of relying on PHP defaults and ensures XML AJAX responses include an explicit charset in the Content-Type header (including early error responses in ajax.php).

Motivation

Special characters and other non-ASCII characters could be rendered incorrectly in multiple places (for example in activity notes), depending on server configuration (such as PHP default_charset) and how XML responses were interpreted by the client.

In these cases, escaping and XML output relied on implicit encodings. Making the encoding explicit stabilizes output across environments and helps prevent character corruption or misrendering.