Onboarding fixes. The welcome screen now presents both paths as two side-by-side cards of equal visual weight — enter a subscriber code, or try Layer on a demo page — instead of a small grey back-arrow link that read as a way to go back rather than an alternative. Also fixes a missing screenshot on the walkthrough screen: the image was referenced by the page but excluded by the build scripts, so users saw an empty frame exactly where they are asked to spot a button.

Each tracking event (install, transfer_complete, target_unavailable) now carries a browser identifier (chrome, firefox, edge, opera, unknown), detected at runtime with no external dependency. Enables Chrome vs Firefox analytics split in the admin dashboard. Backward compatible: pre-3.3.10 events continue to display as before.