fix: fix blocks flashing on mount at zero-zero position

[marshallern]

Problem

When blocks mount at the detailed react view, they flash at 0 0 position before rendering at the correct one.
How to reproduce

1. Set the SCALES parameter in config at [0.15, 0.15, 0.20] so that even at small scales rendered a react component.
2. Open some graph with the lots of blocks
3. Start zooming-in and zooming-out so that new blocks mount

Why it happens

In Block.tsx useSignalEffect is only called after first render because it is run asynchronously. Hence, for a moment you can see blocks at the default positions at 0 0.

Solution

Create another hook that sets the correct block positions that is run before the first render.

Bug screencast: https://github.com/user-attachments/assets/460df84c-e16a-4d00-812c-0cf40a9cc48c

[gravity-ui-bot]

Preview is ready.

[draedful]

Thanks for taking the time to contribute and for the clear write-up — fixes like this really help the library, and we appreciate you digging into the flashing issue.

The approach with useLayoutEffect looks good. One small follow-up: the new effect duplicates the geometry / z-index logic that already lives in useSignalEffect. Would you mind extracting a shared helper (e.g. something like applyBlockContainerLayout(...)) and calling it from both places? That way we keep mount-time and signal-driven updates aligned if this block changes later

[marshallern]

@draedful extracted the common logic and moved it to a separate utils file

[draedful]

Everything is fine, thanks for the contribution.