fix(cli): fetch a page's assets as the same agent that loaded the page (#3726)

A capture is one session with two halves: Chrome navigates the page with a
browser User-Agent, then Node fetches the assets that page referenced. Those
halves sent three different identities — "HyperFrames/1.0" from the asset and
media downloaders, a bare "Mozilla/5.0" from the stylesheet inliner, and the
real Chrome UA from the navigation itself.

An origin is free to answer those differently, and anti-bot edges do. Capturing
one large site, GET /favicon.svg answers 403 text/html to "HyperFrames/1.0" and
200 image/svg+xml to the UA the very same capture had just navigated with. The
favicon ranker had already picked that SVG as the best declared icon; the 403
discarded it and the downloader fell through to the next candidate, so the icon
written to assets/ was chosen by the CDN's bot rules rather than by the ranker.
The capture reported it as one "unavailable" drop and carried on.

Hoist the navigation UA into CAPTURE_USER_AGENT and use it for every
out-of-band fetch the capture makes: favicons, images, og:image, fonts,
stylesheets, Lottie JSON and videos. One constant is what stops the two halves
drifting apart again.

Verified end to end against that site: before, assets/favicon.png (the
apple-touch icon) plus one unavailable drop; after, assets/favicon.svg, byte
identical to the file the site itself serves.