Perform full PHP 8.2–8.5 compatibility sweep

[fiammybe]

Perform a cross-cutting compatibility pass to eliminate deprecated/risky legacy patterns throughout the module.

Checklist:

• Remove dynamic property writes (all declared in advance)
• Remove @ error suppression
• Guard all null/array/string usages
• Harden all serialization/unserialization, replace with JSON if possible
• Remove dead or deprecated code (e.g. browser hacks or old editors)
• Normalize escaping/output (htmlspecialchars) anywhere user data is output
• Run static analyzer and address warnings