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/backups.go

@@ -110,6 +110,16 @@ func (s *Server) handleAdminRunBackup(w http.ResponseWriter, r *http.Request) {
 	s.handleAdminListBackups(w, r)
 }
 
+// handleAdminDeleteBackup removes one database snapshot (admins only; users cannot delete their own
+// backups, those expire with the retention period).
+func (s *Server) handleAdminDeleteBackup(w http.ResponseWriter, r *http.Request) {
+	if err := storage.DeleteBackup(filepath.Join(s.backupRoot(), "system"), chi.URLParam(r, "name")); err != nil {
+		writeError(w, http.StatusNotFound, err.Error())
+		return
+	}
+	s.handleAdminListBackups(w, r)
+}
+
 func (s *Server) handleAdminDownloadBackup(w http.ResponseWriter, r *http.Request) {
 	serveBackupFile(w, r, filepath.Join(s.backupRoot(), "system"), chi.URLParam(r, "name"), "application/gzip")
 }