NewBrowser previously had no viewport (strong headless signal) and used a
Firefox User-Agent unconditionally, even for Chromium instances (detectable
mismatch).
Add per-engine UA constants (DefaultFirefoxUserAgent, DefaultChromiumUserAgent)
and auto-select the matching UA in initBrowser when the caller hasn't set one
explicitly. Keep DefaultUserAgent as a backward-compatible alias.
Add 1920x1080 default viewport to NewBrowser (most common desktop resolution).
NewInteractiveBrowser keeps its existing 1280x720 default but also gains
engine-aware UA selection.
Closes#70
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>