Gate admin routes behind auth and add copy guest link button

When auth is enabled, admin pages require the logged-in user to be
the event owner — unauthorized visitors get redirected to the guest
view, and admin actions return 403. Also adds a copy-to-clipboard
button in the admin bar and a Makefile for common commands.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,13 @@
+.PHONY: build run dev sqlc
+
+build:
+	go build -o bbq .
+
+run: build
+	./bbq
+
+dev: build
+	BBQ_FEATURES=auth ./bbq
+
+sqlc:
+	$(shell go env GOPATH)/bin/sqlc generate