Fix SQL Injection in MainController

[G30RG3-GJ]

Fixed SQL Injection vulnerability in MainController.loadMemberInfo and other methods by using parameterized queries (PreparedStatement).
Added execQuery and execAction methods to DatabaseHandler to support parameters.
Fixed resource loading issue in DatabaseHandler (DatabaseHandler.class.getResourceAsStream vs getClass()).
Ensured PreparedStatement is closed properly to avoid resource leaks.

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PR created automatically by Jules for task 4200065667553345099 started by @G30RG3-GJ

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