* feat(cli-v2): add fern docs diff command

* Improve docs diff auth, screenshots, URL

Refactor docs diff command to improve auth, URL normalization and screenshot handling. Replace askToLogin with getToken which supports self-hosted flows via FERN_TOKEN and falls back to context.getTokenOrPrompt. Use URL parsing to normalize preview host and simplify production URL info handling (fall back on invalid URLs). Add runtime response validation for slug API responses. Pass a Logger into captureScreenshot, improve error logging, and adjust screenshot path typing; update side-by-side comparison call. Reorder pngjs/puppeteer deps in package.json and update pnpm lock entries accordingly.

* fix: use request interception instead of setExtraHTTPHeaders to prevent FERN_TOKEN leaking to third-party domains

* fix: guard request interception handler against malformed URLs

* fix: remove --ignore-certificate-errors and add preview URL domain allowlist

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Co-authored-by: Naman Anand <info@buildwithfern.com>