feat(migration): add v4-to-v5 settings migration runner with backup

src/main/updater/migration-runner.js

@@ -0,0 +1,38 @@
+const fs = require('fs');
+const path = require('path');
+
+class MigrationRunner {
+  constructor({ dir, transform }) {
+    this.dir = dir;
+    this.transform = transform;
+    this.file = path.join(dir, 'settings.json');
+    this.backup = path.join(dir, 'settings.v4.bak.json');
+  }
+
+  run() {
+    if (!fs.existsSync(this.file)) {
+      this._writeDefaults();
+      return 'fresh';
+    }
+    const raw = JSON.parse(fs.readFileSync(this.file, 'utf-8'));
+    if (raw && raw['migration.version'] === 5) {
+      return 'skipped';
+    }
+    try {
+      const v5 = this.transform(raw);
+      fs.copyFileSync(this.file, this.backup);
+      fs.writeFileSync(this.file, JSON.stringify({ ...v5, 'migration.version': 5 }, null, 2));
+      return 'migrated';
+    } catch (err) {
+      console.error('[migration-runner] transform failed:', err.message);
+      return 'failed';
+    }
+  }
+
+  _writeDefaults() {
+    const v5 = this.transform({});
+    fs.writeFileSync(this.file, JSON.stringify({ ...v5, 'migration.version': 5 }, null, 2));
+  }
+}
+
+module.exports = { MigrationRunner };