fix: tutorial scrollymation snapping to Go! section on landing

[EDRipper]

Summary

• fresh accounts landing on /tutorial were getting scroll-locked straight onto the "Go!" section, skipping Join Slack / Connect Hackatime / Create a Project entirely
• reproduced live on the remy ripper test account: clicking Start jumps to Go! immediately
• root cause: the scroll-lock effect (frontend/src/routes/tutorial/+page.svelte) compared against the reactive scrollY binding, which lags the native scroll event. right when locked flips true (synchronously, right after the landing scrollTo), that binding can still read stale/near-zero, so the clamp snapped scroll up to top (the Go! section) instead of resting on the intended landing section (Join Slack)
• fix: read window.scrollY directly for the comparison instead of the lagging binding

Test plan

• click through /tutorial on a fresh/incomplete account and confirm it lands on Join Slack, not Go!
• confirm scroll-locking (can't scroll past top/bottom) still works while stepping through sections