feat(web): chat over an upload opens with App.UploadChatModel

The Chatter's own default is the summarizer's (koala/phi4-mini, 8,192-token
context), which answers HTTP 400 on an hour of speech. Empty keeps the old
behaviour.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/web/uploads.go

@@ -206,7 +206,7 @@ func (a *App) handleUploadChat(w http.ResponseWriter, r *http.Request) {
 		http.Redirect(w, r, "/uploads/"+u.ID, http.StatusSeeOther)
 		return
 	}
-	a.render(w, r, UploadChatPage(*u, a.uploadChatView(*u, a.Chat.DefaultModel(""), nil, false, "")))
+	a.render(w, r, UploadChatPage(*u, a.uploadChatView(*u, a.Chat.DefaultModel(a.UploadChatModel), nil, false, "")))
 }
 
 // handleUploadChatMessage answers one question against the owner's uploaded
@@ -226,7 +226,7 @@ func (a *App) handleUploadChatMessage(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "bad form", http.StatusBadRequest)
 		return
 	}
-	model := a.resolveModel(r.FormValue("model"), "")
+	model := a.resolveModel(r.FormValue("model"), a.UploadChatModel)
 	history := parseHistory(r.Form["hq"], r.Form["ha"])
 	question := strings.TrimSpace(r.FormValue("question"))