Rename quack to quak

German for 'quack', matching the Ente (German for 'duck') naming. All
references updated: package name, CLI binary, X-Client-Package header,
test descriptions, temp dir prefixes, README, Makefile docker tag.

Makefile

@@ -33,7 +33,7 @@ clean:
 	@rm -rf dist coverage .vitest-cache *.tsbuildinfo
 
 docker:
-	docker build -t quack .
+	docker build -t quak .
 
 hooks:
 	@printf '#!/bin/sh\nset -e\nmake lint\nmake fmt-check\n' > .git/hooks/pre-commit