perf(website): add rel=preload for gosqlx.wasm on /playground

[ajitpratap0]

Problem

The 6.5 MB WASM binary starts downloading only after JS hydration on the /playground page, delaying interactive time significantly. The load waterfall is:

1. Page hydrates → JS evaluates → initWasm() called
2. loadScript("/wasm/wasm_exec.js") fetched (sequential)
3. fetch("/wasm/gosqlx.wasm") — 6.2 MB (sequential, no preload)
4. WebAssembly.instantiate() called

This makes the playground unusable for several seconds on first load, especially on mobile.

Fix

Add to the playground page: <link rel='preload' as='fetch' href='/wasm/gosqlx.wasm' crossOrigin='anonymous'> — this starts the WASM download in parallel with JS execution, cutting time-to-interactive by 1–3 seconds on fast connections and 5–15 seconds on mobile.