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>

server/routes/weather.js

@@ -105,7 +105,10 @@ router.get('/', async (req, res) => {
         humidity:   currentJson.main.humidity,
         icon:       currentJson.weather[0]?.icon,
         desc:       currentJson.weather[0]?.description,
-        wind_speed: Math.round((currentJson.wind?.speed ?? 0) * 3.6), // m/s → km/h
+        // metric/standard: m/s → km/h; imperial: already mph
+        wind_speed: units === 'imperial'
+          ? Math.round(currentJson.wind?.speed ?? 0)
+          : Math.round((currentJson.wind?.speed ?? 0) * 3.6),
       },
       forecast: forecastDays,
     };