search: cache index per locale and tighten iframe postMessage origin

[avinxshKD]

This changes the search flow to fetch/build the index only when locale changes, reuse the same Fuse instance for typing with a small debounce, and abort stale fetches, and it also removes wildcard postMessage usage by using same-origin target/checks.
I’m sending this as a small perf/safety cleanup for #1249 since the p5.sound part looks outdated now, but the repeated search work and loose messaging behavior were still valid to fix.

Refs #1249

[avinxshKD]

@ksen0 @doradocodes pls take a look, kept focused fixes as discussed (search fetch/index flow + postMessage origin hardening)

[doradocodes]

@avinxshKD From my understanding, this PR includes 2 different fixes:

1. Fixes the issue with SearchProvider that rebuilds even when locale hasn't changed
2. The use of * in postMessage() for iframes

If that is the case, it's important that we have 2 different issues created and 2 separate PRs to fix them. This helps keep things organized for the repo. Please update the original issue #1249 to have an accurate title+description, and create a new issue if relevant. Then have 2 PRs for the each of those issues. Thanks!