Name the item and what is lost in each delete prompt (closes #400)

The webhook, entrypoint and target delete prompts on the webhook page
now name the item and say what deleting it loses: the webhook's stored
events (with the count from the statistics pane) and their deliveries,
while its archive files are kept; an entrypoint's URL, which stops
working for good; a target's future deliveries, while its past ones
stay in the event log. They stay the browser's own prompts, so they
work without the page's scripts, and each name is escaped for the
script so quotes and backslashes show as typed.

Model: opus-5-5

internal/handlers/source_management.go

@@ -612,8 +612,9 @@ func (h *Handlers) renderSourceDetail(
 
 	// The host is the client's Host header, unvalidated. It is
 	// inert only because source_detail.html renders BaseURL as
-	// text inside a <code> element; putting it in an href or any
-	// other URL context needs it constrained first.
+	// text, inside a <code> element and in an entrypoint's delete
+	// prompt; putting it in an href or any other URL context
+	// needs it constrained first.
 	baseURL := scheme + "://" + r.Host
 
 	// The template calls Webhook methods, which take pointer