Show each page's own title in the browser tab (closes #117)

parsePageTemplate parsed the page file before htmlheader.html. A later
definition of a template name replaces an earlier one (an empty one
never does), so htmlheader.html's {{block "title"}} replaced every
page's {{define "title"}} and every tab read "Webhooker". The page file
is now parsed last, into a set named after it, so it stays the entry
point.

A new test renders every page template and checks its title; the
"Webhooker" fallback is checked on htmlheader.html alone, since every
page declares a title.

Model: opus-5-5

internal/handlers/handlers.go

@@ -110,22 +110,25 @@ type Handlers struct {
 // parsePageTemplate parses a page-specific template set from the
 // embedded FS. Each page template is combined with the shared
 // base, htmlheader, navbar and notice templates, and with any further
-// files the page includes. The page file must be listed first so that
-// its root action ({{template "base" .}}) becomes the template set's
-// entry point.
+// files the page includes. The set is named after the page file, so
+// the page's root action ({{template "base" .}}) is its entry point.
+//
+// The page file is parsed last because a later definition of a name
+// replaces an earlier one: the page's {{define "title"}} must replace
+// the {{block "title"}} fallback in htmlheader.html.
 func parsePageTemplate(
 	pageFile string, included ...string,
 ) *template.Template {
 	files := append([]string{
-		pageFile,
 		"base.html",
 		"htmlheader.html",
 		"navbar.html",
 		"notice.html",
 	}, included...)
+	files = append(files, pageFile)
 
 	return template.Must(
-		template.ParseFS(templates.Templates, files...),
+		template.New(pageFile).ParseFS(templates.Templates, files...),
 	)
 }