Settings validator accepts whitespace-padded numbers the engine can't parse

[genwave-radio]

🐛 Problem

The settings validator accepts a number with surrounding whitespace, but the engine reads the same value as invalid. The saved setting then has no effect, and nothing tells the operator.

• SettingValidator.IsDoubleInRange / IsDoubleAboveAndAtMost (src/GenWave.Host/Configuration/SettingValidator.cs) parse with NumberStyles.Float, which allows leading and trailing whitespace. So "3 " passes validation and is saved.
• In the engine container, Liquidsoap's float_of_string("3 ") fails, so genwave.liq falls back to the default.

Result: the operator saves GW_XFADE_MIN = "3 ", gets a success, and the engine keeps running 2.0. Once F213 (gh-#879) ships, /api/status will also report a restartNeeded that no restart can clear.

The problem was found while reviewing T601, part of F213.

✅ Fix options

1. Validator (preferred). Parse the engine-bound doubles with NumberStyles.AllowLeadingSign | AllowDecimalPoint | AllowExponent, so whitespace is rejected with a 400.
2. Trim on save, so the stored value matches what the engine can parse.

Either way, add a spec: saving "3 " gives a 400, or it is stored as "3".

The affected keys are every number setting served by /internal/engine-config: GW_XFADE_MIN, GW_XFADE_MAX and GW_SAFE_GAP_SECONDS.