chore(clerk-js): Migrate to rspack for building

.changeset/bright-ducks-enjoy.md

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+---
+"@clerk/clerk-js": patch
+---
+
+`clerk-js` is now built with [Rspack](https://rspack.dev) instead of Webpack. This is an internal tooling change that shouldn't affect consumers.

.changeset/cool-days-tell.md

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+---
+"@clerk/elements": patch
+---
+
+Use host router instead of directly calling Next's `useRouter` hook