fix: date timezone + weather periodic refresh (closes #5, #7)

- Use localTime instead of epochTime when formatting date string,
  so date rolls over at local midnight, not UTC midnight (fixes #5)
- Remove WEATHER_SUCCESS state assignment that blocked periodic refresh;
  state correctly stays WEATHER_IDLE after successful fetch (fixes #7)
- Fix weather debug page "Last update" to show elapsed seconds
  (millis() - lastUpdate) instead of raw timestamp

Applied from PR #6 by @Mysteoa with a compilation fix in web_server.cpp
(missing closing parenthesis in snprintf_P call).

Co-authored-by: Mysteoa <mysteoa@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/clock_ntp_ota_v1.9/weather.cpp

@@ -83,7 +83,7 @@ void ICACHE_FLASH_ATTR onWeatherResponse(void* optParm, AsyncHTTPRequest* reques
           sunTimes.lastDay = ptm->tm_yday;
         }
 
-        weatherState = WEATHER_SUCCESS;
+        // weatherState stays WEATHER_IDLE (set at readyState==4 entry) — allows periodic refresh
         weatherRetry.reset();
         Serial.printf("Weather: %.1f C, code %d, wind %.1f km/h\n",
                       weather.temperature, weather.weathercode, weather.windspeed);