Unify number slider threshold across rows and state cards

[piitaya]

Proposed change

Three places decide whether a number entity in auto mode renders a slider or a text box, and one of them gets it wrong. state-card-number compares max - min against 256, the other two compare (max - min) / step.

So a number with min 0, max 1000, step 10 renders a slider in the Lovelace row and the config flow preview, and a text box in the state card, for 100 steps in every case.

Extracted showNumberSlider into src/data/number.ts and used it at all three sites, rather than spelling the formula a third time.

This changes what the state card renders for entities whose range is above 256 but whose step count is not, so it is a visible change worth a look. The row and the preview are unaffected, their behaviour is what the state card now matches.

Screenshots

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• There is no commented out code in this PR.
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