refactor: use extensible design for fetching playlist from single file

app/playlistscouts/qtscout.cpp

@@ -0,0 +1,40 @@
+// SPDX-FileCopyrightText: 2025 Gary Wang <git@blumia.net>
+// SPDX-FileCopyrightText: 2026 yyc12345 <yyc12321@outlook.com>
+//
+// SPDX-License-Identifier: MIT
+
+#include "qtscout.h"
+
+#include <QFileInfo>
+#include <QDir>
+#include <QCollator>
+
+std::optional<PlaylistScout::ScoutResult> PlaylistScout::Qt::scout(
+    const QUrl &url, const QStringList &allowedSuffixes) {
+    PlaylistScout::ScoutResult rv;
+
+    QFileInfo info(url.toLocalFile());
+    QDir dir(info.path());
+    QString currentFileName = info.fileName();
+
+    QStringList entryList = dir.entryList(
+        allowedSuffixes, QDir::Files | QDir::NoSymLinks, QDir::NoSort);
+
+    QCollator collator;
+    collator.setNumericMode(true);
+
+    std::sort(entryList.begin(), entryList.end(), collator);
+
+    for (qsizetype i = 0; i < entryList.count(); i++) {
+        const QString &fileName = entryList.at(i);
+        const QString &oneEntry = dir.absoluteFilePath(fileName);
+        const QUrl &url = QUrl::fromLocalFile(oneEntry);
+
+        rv.playlist.append(url);
+        if (fileName == currentFileName) {
+            rv.indexHint = i;
+        }
+    }
+
+    return rv;
+}