Add Distinta Base import tab: parse .xls/.xml export to CSV with preview

Reads the ERP-exported "Distinta Base" file (SpreadsheetML XML or real
binary .xls/.xlsx via ExcelDataReader), extracts Componente/Descrizione,
writes a CSV and previews it in a new tab alongside the existing CAD
search. Also ignore the exported Excel/CSV files dropped in the repo root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CadSearchGui/Form1.cs

@@ -15,6 +15,8 @@ public partial class Form1 : Form
     private readonly DataGridView _grid = new();
     private readonly Label _lblStatus = new();
     private readonly Label _lblDb = new();
+    private readonly DataGridView _gridImport = new();
+    private readonly Label _lblImportStatus = new();
 
     private const string OpzionePrtAsm = "Solo prt/asm (predefinito)";
     private const string OpzioneTuttiTipi = "Tutti i tipi";
@@ -120,9 +122,96 @@ public partial class Form1 : Form
         _lblStatus.Text = "Pronto.";
         statusPanel.Controls.Add(_lblStatus);
 
-        Controls.Add(_grid);
-        Controls.Add(statusPanel);
-        Controls.Add(topPanel);
+        var tabRicerca = new TabPage("Ricerca CAD");
+        tabRicerca.Controls.Add(_grid);
+        tabRicerca.Controls.Add(statusPanel);
+        tabRicerca.Controls.Add(topPanel);
+
+        var tabs = new TabControl { Dock = DockStyle.Fill };
+        tabs.TabPages.Add(BuildTabImportaDistinta());
+        tabs.TabPages.Add(tabRicerca);
+
+        Controls.Add(tabs);
+    }
+
+    private TabPage BuildTabImportaDistinta()
+    {
+        var tab = new TabPage("Importa Distinta Base") { AllowDrop = true };
+
+        var lblIstruzioni = new Label
+        {
+            Dock = DockStyle.Top,
+            Height = 60,
+            Padding = new Padding(8),
+            Text = "Trascina qui il file .xls/.xml esportato dal gestionale (foglio 'Distinta "
+                 + "Base'), oppure scegli il file col pulsante sotto. Verrà generato un CSV con "
+                 + "le colonne Componente e Descrizione, con anteprima qui sotto.",
+        };
+
+        var btnSfoglia = new Button { Text = "Scegli file...", Dock = DockStyle.Top, Height = 32 };
+        btnSfoglia.Click += (_, _) => ScegliFileDistinta();
+
+        _lblImportStatus.Dock = DockStyle.Top;
+        _lblImportStatus.Height = 24;
+        _lblImportStatus.Padding = new Padding(8, 4, 8, 4);
+        _lblImportStatus.ForeColor = SystemColors.GrayText;
+
+        _gridImport.Dock = DockStyle.Fill;
+        _gridImport.ReadOnly = true;
+        _gridImport.AllowUserToAddRows = false;
+        _gridImport.AllowUserToDeleteRows = false;
+        _gridImport.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.Fill;
+        _gridImport.Columns.Add("componente", "Componente");
+        _gridImport.Columns.Add("descrizione", "Descrizione");
+
+        tab.Controls.Add(_gridImport);
+        tab.Controls.Add(_lblImportStatus);
+        tab.Controls.Add(btnSfoglia);
+        tab.Controls.Add(lblIstruzioni);
+
+        tab.DragEnter += (_, e) =>
+            e.Effect = e.Data?.GetDataPresent(DataFormats.FileDrop) == true
+                ? DragDropEffects.Copy
+                : DragDropEffects.None;
+        tab.DragDrop += (_, e) =>
+        {
+            if (e.Data?.GetData(DataFormats.FileDrop) is string[] { Length: > 0 } file)
+                ImportaDistinta(file[0]);
+        };
+
+        return tab;
+    }
+
+    private void ScegliFileDistinta()
+    {
+        using var dialog = new OpenFileDialog
+        {
+            Filter = "File distinta base (*.xls;*.xml)|*.xls;*.xml|Tutti i file|*.*",
+            Title = "Seleziona il file Distinta Base",
+        };
+        if (dialog.ShowDialog(this) == DialogResult.OK)
+            ImportaDistinta(dialog.FileName);
+    }
+
+    private void ImportaDistinta(string percorso)
+    {
+        try
+        {
+            var (csv, righe) = DistintaBaseImporter.ConvertiInCsv(percorso);
+            _gridImport.Rows.Clear();
+            foreach (var (componente, descrizione) in righe)
+            {
+                _gridImport.Rows.Add(componente, descrizione);
+            }
+            _lblImportStatus.ForeColor = Color.DarkGreen;
+            _lblImportStatus.Text = $"CSV generato ({righe.Count} righe): {csv}";
+        }
+        catch (Exception ex)
+        {
+            _gridImport.Rows.Clear();
+            _lblImportStatus.ForeColor = Color.DarkRed;
+            _lblImportStatus.Text = $"Errore durante la conversione: {ex.Message}";
+        }
     }
 
     private void InizializzaDatabase()