Add explicit working folder; rename project from CadSearchGui to PrepStep

Working folder (phase 1): CSV/raw/step/step_finali all used to live split
across two different implicit locations (CSV in the "repo root" found by
walking up for CLAUDE.md, raw/step next to the input distinta file) with
neither shown to the user. That repo-root lookup also silently breaks once
the app runs as a standalone published exe on a machine with no repo/
CLAUDE.md at all. Replaced both with one explicit "cartella di lavoro",
auto-suggested (last used, remembered the same way as the DB path) and
changeable from phase 1, with no dependency on any repo marker file.

Rename: CadSearchGui was the working name from before the app had a
product identity; renamed folders, csproj files, namespace and the
AppData settings folder to PrepStep throughout, matching the exe name
already in use. Product-facing name stays "Cevolani StepPrep".

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

PrepStep.Tests/DistintaBaseImporterTests.cs

@@ -0,0 +1,69 @@
+using PrepStep;
+
+namespace PrepStep.Tests;
+
+public class DistintaBaseImporterTests
+{
+    private const string SpreadsheetMlMinimo = """
+        <?xml version="1.0"?>
+        <Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet">
+          <Worksheet ss:Name="Distinta Base" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet">
+            <Table>
+              <Row><Cell ss:Index="2"><Data>Componente</Data></Cell><Cell><Data>Descrizione</Data></Cell></Row>
+              <Row><Cell ss:Index="2"><Data>1234567890</Data></Cell><Cell><Data>Vite</Data></Cell></Row>
+            </Table>
+          </Worksheet>
+        </Workbook>
+        """;
+
+    private static string ScriviFileDistinta(string nomeFile, string contenuto)
+    {
+        string percorso = Path.Combine(Path.GetTempPath(), $"cadsearch_test_{Guid.NewGuid():N}", nomeFile);
+        Directory.CreateDirectory(Path.GetDirectoryName(percorso)!);
+        File.WriteAllText(percorso, contenuto);
+        return percorso;
+    }
+
+    [Fact]
+    public void Aggiunge_assieme_generale_dedotto_dal_nome_file_DBP()
+    {
+        string percorso = ScriviFileDistinta("DBP_9244228573_20260916144238.xls", SpreadsheetMlMinimo);
+        try
+        {
+            var (_, righe) = DistintaBaseImporter.ConvertiInCsv(percorso);
+
+            Assert.Contains(righe, r => r.Componente == "9244228573" && r.Descrizione == "Assieme generale");
+            Assert.Contains(righe, r => r.Componente == "1234567890");
+        }
+        finally { Directory.Delete(Path.GetDirectoryName(percorso)!, recursive: true); }
+    }
+
+    [Fact]
+    public void Non_duplica_lassieme_generale_se_gia_presente_tra_i_componenti()
+    {
+        // Caso limite: il codice del nome file coincide con un componente già elencato nel foglio.
+        string contenutoConGeneraleGiaPresente = SpreadsheetMlMinimo.Replace("1234567890", "9244228573");
+        string percorso = ScriviFileDistinta("DBP_9244228573_20260916144238.xls", contenutoConGeneraleGiaPresente);
+        try
+        {
+            var (_, righe) = DistintaBaseImporter.ConvertiInCsv(percorso);
+
+            Assert.Single(righe, r => r.Componente == "9244228573");
+        }
+        finally { Directory.Delete(Path.GetDirectoryName(percorso)!, recursive: true); }
+    }
+
+    [Fact]
+    public void Nessun_assieme_generale_aggiunto_se_il_nome_file_non_segue_il_pattern_DBP()
+    {
+        string percorso = ScriviFileDistinta("estrazione_qualsiasi.xls", SpreadsheetMlMinimo);
+        try
+        {
+            var (_, righe) = DistintaBaseImporter.ConvertiInCsv(percorso);
+
+            Assert.DoesNotContain(righe, r => r.Descrizione == "Assieme generale");
+            Assert.Single(righe);
+        }
+        finally { Directory.Delete(Path.GetDirectoryName(percorso)!, recursive: true); }
+    }
+}