feat: Kanban touch drag, admins delete backups, bigger share message

- Kanban on phones/tablets: hold a card, drag it onto a card or column, release; the page scrolls at the edges
- Admin settings: database backups can be deleted (DELETE /api/v1/admin/backups/{name})
- Share message: large Liste/Aufgabe buttons, close button, tap outside closes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -303,6 +303,7 @@ func (s *Server) Router() http.Handler {
 			r.Get("/api/v1/admin/backups", s.handleAdminListBackups)
 			r.Post("/api/v1/admin/backups", s.notInDemo(s.handleAdminRunBackup))
 			r.Get("/api/v1/admin/backups/{name}", s.handleAdminDownloadBackup)
+			r.Delete("/api/v1/admin/backups/{name}", s.notInDemo(s.handleAdminDeleteBackup))
 			r.Put("/api/v1/admin/settings", s.notInDemo(s.handleAdminUpdateSettings))
 			r.Get("/api/v1/admin/settings/sharing-impact", s.handleAdminSharingImpact)
 			r.Get("/api/v1/admin/voice", s.handleAdminVoiceStatus)