feat: serve the student app from the plugin

plugin.go

@@ -46,6 +46,9 @@ var adminFS embed.FS
 // (Bezpieczeństwo międzynarodowe) master's programme.
 type Plugin struct {
 	app *backpack.App
+	// publicFS is the tree of the student app. Nil means the embedded
+	// public/ directory; tests plant their own.
+	publicFS fs.FS
 }
 
 func (p *Plugin) ID() string { return "jz.bm" }