Modernize admin UX to ImpressCMS 2 conventions

[fiammybe]

Refactor the admin area to use more consistent ImpressCMS 2 patterns and IPF-friendly forms/tables.

Checklist:

• Refactor admin menu and flow to share more code via a common bootstrap/controller
• Use IPF forms/tables/tree displays for core entities
• Standardize admin messages, redirects, and page layout
• Confirm all admin CRUD actions (category/forum/report, etc.) work as before