admin-ui: Station:Location lat/long give no format help and invalid values silently disable weather

[genwave-radio]

🐛 Symptom (T229, demo box)

Station:Location:Latitude / Longitude help text says nothing about format. Operator question from testing: "Is it DD, DMS, DMM, all of them?"

📌 The actual contract (from code)

WeatherContextProvider.TryParseCoordinates (WeatherContextProvider.cs:217-232):

• Signed decimal degrees only (double.TryParse, InvariantCulture). DMS (53°32'N), DMM, and comma-decimal (53,54) all fail.
• Ranges: lat ±90, lon ±180. Negative = S/W.
• Only ≤4 decimal places reach Open-Meteo (formatted "0.####", :253-258).
• Invalid value ≡ blank: weather silently fails closed (F108.1) with a single Information log. Save-time validation is deliberately AlwaysValid (SettingValidator.cs:372-377) — so an operator who enters DMS gets a green save and a weatherless station with no visible reason.

🔧 Fix sketch

1. Help text (SettingsForm.tsx:349-354): state signed decimal degrees, ranges, sign convention, period decimal separator, 4-dp precision, and that invalid ≡ blank (weather off).
2. Optional, worth it: client-side soft warning when the value is non-blank and doesn't parse as DD in range — keep the server permissive as designed, just stop the silent-footgun at the form.

✅ Acceptance

• Hover help answers the DD/DMS question outright.
• Entering 53°32'N produces an inline warning before save (if item 2 is taken).