Flutter: tampilkan URL server tersimpan saat layar login berikutnya

- Session.clear() kini hapus sesi (user/pass) tapi sisakan URL terakhir
- + Session.readLastUrl()
- LoginScreen mengisi field URL dari URL terakhir saat dibuka
- sesudah logout login lagi URL sudah terisi, tak perlu ketik ulang

flutter/lib/services/session.dart

@@ -27,9 +27,12 @@ class Session {
   }
 
   static Future<void> clear() async {
-    await _storage.delete(key: _kBase);
-    await _storage.delete(key: _kDb);
+    // Hapus sesi (user & pass), tapi sisakan URL (_kBase) agar login berikutnya mudah
     await _storage.delete(key: _kUser);
     await _storage.delete(key: _kPass);
   }
+
+  static Future<String?> readLastUrl() async {
+    return _storage.read(key: _kBase);
+  }
 }