feat(cli): auth flows via Playwright storage state

[nucliweb]

Goal

Allow measuring pages that require authentication by passing a Playwright storage state file (cookies + localStorage), using Playwright's own native format.

Usage

# Step 1 — capture auth state with Playwright codegen
npx playwright codegen --save-storage=auth.json https://example.com/login

# Step 2 — measure an authenticated page
npx webperf-snippets https://example.com/dashboard --storage-state auth.json

Implementation

One-line change in cli/src/runner.js:

// before
const context = await browser.newContext({ viewport });

// after
const context = await browser.newContext({ viewport, storageState: opts.storageState });

Files

• cli/src/runner.js — pass storageState to newContext()
• cli/src/bin.js — expose --storage-state <path> flag

Acceptance criteria

• --storage-state <path> restores session before navigation
• Fails with a clear error if the path does not exist
• Documented in README with the full two-step workflow