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>

scripts/ExtractCreoStrings.cs

@@ -3,7 +3,7 @@
 //
 // Estrae dal file .asm i codici dei componenti referenziati, espandendo ricorsivamente
 // ogni sotto-assieme (.asm) trovato nell'indice, finché non restano solo PRT (foglie) o
-// codici non presenti nell'indice. Stessa euristica di CadSearchGui/AssemblyComponentScanner.cs:
+// codici non presenti nell'indice. Stessa euristica di PrepStep/AssemblyComponentScanner.cs:
 // i codici veri compaiono più volte nel file, il rumore binario quasi sempre una sola volta;
 // gli ID interni "ModelView#<numero>" sono esclusi via lookbehind - vedi scripts/Analyze.cs.
 // Ogni codice trovato viene validato contro l'indice cad_index.db.