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/Program.cs

@@ -0,0 +1,16 @@
+namespace PrepStep;
+
+static class Program
+{
+    /// <summary>
+    ///  The main entry point for the application.
+    /// </summary>
+    [STAThread]
+    static void Main()
+    {
+        // To customize application configuration such as set high DPI settings or default font,
+        // see https://aka.ms/applicationconfiguration.
+        ApplicationConfiguration.Initialize();
+        Application.Run(new Form1());
+    }    
+}