New loading before rendering

[Marc-Andrieu]

Fix #47: see my comment there.

Content of this PR:

• Putting the fallback of <SuspenseEmbed> to <></>: no annoying "Loading..." anymore.
• Adding pure CSS skeletons that match perfectly ("at least on my machine") the shape of the <FormLabel> and <FormControl> of the <LoginCustomFormField>: this way, there's no more dependency and the size of the <CenteredCard> is preserved.
• Version +2 because I forgot to update it in my previous PR.

We may want some refactoring later for the CSS and the <Suspense>-derived tags...

[Marc-Andrieu]

How the skeletons render on my phone:

• I artificially made them last a bit longer here for the video's sake.
• If you zoom and use slow motion, it's imperceptibly taller with the skeletons.

https://github.com/user-attachments/assets/9a91f0ef-18bb-4e5d-9a45-1301b30ce4b4

[Marc-Andrieu]

Rendering on phone (see above for comparison): imperceptibly shorter this time

https://github.com/user-attachments/assets/ec1d4df9-83c1-47ad-a228-1f3b08761895