T5 products in the client's inventory

The inventory's product list (test.showInventory) gets the community products the person has: local packs, and
hub products in their library. Hiding one is answered by the proxy (and removes a hub product from the library).
When the library changes, the client is told to load its inventory again, with the server event IMVU sends
after a purchase. A product whose revision or info changes gets a new local ID, since the client keeps product
info for good; names lose characters the client's pages could take as HTML.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/proxy/proxy.go

@@ -374,3 +374,18 @@ func copyFlushing(w http.ResponseWriter, body io.Reader) (int64, error) {
 func since(t time.Time) string {
 	return time.Since(t).Round(time.Millisecond).String()
 }
+
+// Interceptors runs several Interceptors in order, stopping at the first that answers the request itself.
+type Interceptors []Interceptor
+
+func (is Interceptors) Intercept(x *Exchange) error {
+	for _, i := range is {
+		if err := i.Intercept(x); err != nil {
+			return err
+		}
+		if x.Response != nil {
+			return nil
+		}
+	}
+	return nil
+}