Replace keys with translatable friendly names in Statistics helper

[NoRi2909]

Proposed change

The Statistics helper UI still uses the internal key names for some field descriptions and error messages.
As these cannot be translated they are not helpful in localized UIs:

This PR fixes this by replacing them with the friendly names in the dialog, all capitalized to recognize them as field labels.

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[home-assistant]

Hey there @ThomDietrich, @gjohansson-ST, mind taking a look at this pull request as it has been labeled with an integration (statistics) you are listed as a code owner for? Thanks!

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[gjohansson-ST]

Why can they not be translated? I think they can, but we seem not to do it?

[NoRi2909]

Well, anything with underscores like max_age, sampling_size and keep_last_sample will not be translated in Lokalise.

But when we use the field labels from the UI like 'Max age', 'Sampling size' and 'Keep last sample' this mandates a translation so it matches the rest of the UI seen by the user in his or her language.

We already applied a bunch of similar fixes to other integrations to ensure proper translations.

[gjohansson-ST]

That's what I sort of meant. That if we put in proper text, then the friends who translate would do this also for these field names?

[NoRi2909]

As I'm doing the translations for German I can assure you that we'll do that here. :-)

For the other languages I cannot tell you how much cross-checking is done there.
But the first step is to get rid of the key names that prevent proper translations.

[gjohansson-ST]

Thanks @NoRi2909 👍