feat: thumbnail picker custom per selezione modello/scena

- GET /folder_image/{filename}?w=N: PNG ridotto cache 1h
- Frontend: 2 thumb-picker al posto dei select (thumb + nome + caret)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pm2d/web/server.py

@@ -292,6 +292,25 @@ def index():
     return HTMLResponse(html_path.read_text(encoding="utf-8"))
 
 
+@app.get("/folder_image/{filename}")
+def folder_image(filename: str, w: int = 120):
+    """Serve thumbnail PNG dell'immagine IMAGES_DIR (scalata a width w)."""
+    if "/" in filename or ".." in filename:
+        raise HTTPException(400, "nome non valido")
+    path = IMAGES_DIR / filename
+    if not path.is_file():
+        raise HTTPException(404, "non trovato")
+    img = cv2.imread(str(path), cv2.IMREAD_COLOR)
+    if img is None:
+        raise HTTPException(400, "non leggibile")
+    h0, w0 = img.shape[:2]
+    if w0 > w:
+        sc = w / w0
+        img = cv2.resize(img, (w, int(h0 * sc)), interpolation=cv2.INTER_AREA)
+    return Response(_encode_png(img), media_type="image/png",
+                    headers={"Cache-Control": "public, max-age=3600"})
+
+
 @app.get("/images")
 def list_images():
     """Lista file immagine nella cartella configurata in IMAGES_DIR."""