fix(leads): fix undefined lang variable in quote validation error path

Used g.get("lang", "en") consistent with the rest of the file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/padelnomics/leads/routes.py

@@ -302,7 +302,7 @@ async def quote_request():
                 "quote_request.html",
                 data=form_data,
                 step=9,
-                steps=_get_quote_steps(lang),
+                steps=_get_quote_steps(g.get("lang", "en")),
                 errors=field_errors,
             )