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/storage/backup.go

@@ -54,6 +54,15 @@ func BackupPath(dir, name string) (string, error) {
 	return p, nil
 }
 
+// DeleteBackup removes one backup in dir; like BackupPath it only accepts our own file names.
+func DeleteBackup(dir, name string) error {
+	p, err := BackupPath(dir, name)
+	if err != nil {
+		return err
+	}
+	return os.Remove(p)
+}
+
 // ListBackups lists the backups in dir, newest first.
 func ListBackups(dir string) ([]BackupFile, error) {
 	entries, err := os.ReadDir(dir)