Dashboard: Navigation Redesign, Organization Settings, Slug Rename

The sidebar is rebuilt from scratch, settings are reorganized, and organizations can now be renamed or deleted directly in the app.

• Sidebar org switcher: dropdown showing all organizations with "Add organization" at the bottom
• Integrations renamed to Connection, so Ghost CMS and each payment provider have their own page
• New Organization settings: edit your org slug with a warning about webhook URL changes
• Danger Zone: delete your organization with slug confirmation (owner only)