feat: add SFTP host key management and security improvements

- Add HTTPS URL validation for extension store registry and downloads
- Add Reset SFTP Host Key button (per-server)
- Add Reset All SFTP Host Keys button
- Add SFTP host key verification with TOFU (Trust On First Use)
- Update cloud upload service with host key storage
- Add flutter_secure_storage dependency for secure password storage

android/app/src/main/AndroidManifest.xml

@@ -20,8 +20,7 @@
         android:label="SpotiFLAC"
         android:name="${applicationName}"
         android:icon="@mipmap/ic_launcher"
-        android:requestLegacyExternalStorage="true"
-        android:usesCleartextTraffic="true"
+        android:usesCleartextTraffic="false"
         android:enableOnBackInvokedCallback="true">
 
         <activity