fix(weather): show correct wind speed and unit for imperial units

When OPENWEATHER_UNITS=imperial, OpenWeatherMap returns wind speed in
mph directly — the server was incorrectly multiplying by 3.6 (m/s→km/h)
on top of that. All locale strings also hardcoded the unit label instead
of using a {{windUnit}} placeholder, so the label always read km/h.

Resolves #79

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/locales/de.json

@@ -72,7 +72,7 @@
     "weatherRefresh": "Wetter aktualisieren",
     "weatherRefreshTitle": "Aktualisieren",
     "weatherUpdated": "Wetter aktualisiert",
-    "weatherFeelsLike": "Gefühlt {{temp}}° · {{humidity}}% · Wind {{wind}} km/h",
+    "weatherFeelsLike": "Gefühlt {{temp}}° · {{humidity}}% · Wind {{wind}} {{windUnit}}",
     "fabTaskLabel": "Aufgabe hinzufügen",
     "fabCalendarLabel": "Termin hinzufügen",
     "fabShoppingLabel": "Einkauf hinzufügen",