feat(speedtest): prefill validation + logos polish

[Flotapponnier]

1. Prefilled endpoints are validated FROM THE BROWSER right at chain pick (parallel eth_chainId, CORS-rejections dropped before the test with a one-line note) — a benchmarked endpoint can never appear as a dead no-CORS tile. During the test, any blocked endpoint is removed from the gauge grid (compact skipped note); the curl fallback card only shows for manually-typed URLs. 2) Directory regenerated with provider slugs; ProviderLogo (same artwork as the leaderboards, initials fallback) on chain chips, search dropdown, gauge tiles, winner banner and result rows.