fix(playwright): install chromium-headless-shell for headless runs

Playwright v1.46 expects the separate chromium_headless_shell binary in
headless mode. Install chromium-headless-shell alongside chromium in both
the Makefile and Nix flake installer to prevent ENOENT launch errors.

https://chatgpt.com/share/694ae842-1588-800f-9418-31e7d02ac45e

Makefile

@@ -83,7 +83,7 @@ deps-e2e: venv
 	@$(VENV_PIP) install -e ".[e2e]"
 
 playwright-install: deps-e2e
-	@$(VENV_PY) -m playwright install chromium
+	@$(VENV_PY) -m playwright install chromium chromium-headless-shell
 
 e2e-up:
 	$(COMPOSE) up -d

flake.nix

@@ -64,7 +64,7 @@
           text = ''
             # Install Playwright browsers.
             # IMPORTANT: Do not print anything to stdout (tests expect token-only stdout).
-            exec ${pythonPlaywright}/bin/python -m playwright install chromium 1>&2
+            exec ${pythonPlaywright}/bin/python -m playwright install chromium chromium-headless-shell 1>&2
           '';
         };
       in