Fix: Apply the default team automatically when creating a panel

[biast12]

Description

The Support Teams picker on /manage/:guildId/panels/create starts empty. Nothing forces a choice — validateTeams returns early on an empty array and default_team is unvalidated — so a panel saved without opening the Routing section is stored with default_team = false and no panel_teams rows.

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

Try to create a newww panel

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works