fix: call setVisible(true) on SessionGUI instance to display the panel

[AmlanDalai]

Bug Fix

Problem

In GuiPanel.java, when the "View Sessions" button was clicked,
a new SessionGUI instance was created but never made visible:

// Before (buggy)
new SessionGUI(projectManager); // panel never shown to user

Fix

Store the instance and call setVisible(true) so the panel
actually appears when the button is clicked:

// After (fixed)
SessionGUI sessionGUI = new SessionGUI(projectManager);
sessionGUI.setVisible(true);

Impact

Users clicking "View Sessions" button would see nothing happen.
This fix makes the Sessions panel actually appear as expected.