Rewrite README: position as real-time Go dashboards, not generic WASM framework

[mirkobrombin]

The README sells "build web applications with WebAssembly" but that's a feature, not a problem anyone has. The real pitch is "Phoenix LiveView for Go" -- SSC + WebSocket + single-language Go end-to-end.

Plan:

• New tagline: "Real-time dashboards and internal tools, written entirely in Go. No JavaScript. No glue code."
• GIF of counter + live update in the hero section
• 8-line quick-start using go install (not curl pipe bash)
• "Why rfw" section before "Getting Started" -- call out templ+HTMX/Datastar honestly
• Link to a live demo hosted on Fly.io
• Move "WebAssembly" mention to a footnote; lead with live data, type safety, real-time

The README currently reads like it was written for someone already inside the project. A visitor from HN needs to get it in 30 seconds.