Improve SvelteKit client navigation spans

### Problem Statement

I noticed a while ago, that navigation spans are extremely short sometimes (1-5ms) which makes me a bit suspicious if we're not missing some spans (e.g. resource spans or similar).

### Solution Brainstorm

* investigate with better e2e test routes
* perhaps stop directly ending the navigation span on the actual end of the navigation but rely on the idle span ending behaviour instead?
* ensure multiple quick subsequent navigations are still treated as separate traces and spans.