fix: C1 C2 H1 H2 H3 — stability fixes for long-running operation

C1 (critical): Add 15s watchdog for WEATHER_REQUESTING state — resets to
IDLE if TCP connection hangs silently, preventing permanent weather death

C2 (critical): Fix millis() rollover in all retry timers — replace unsafe
`millis() >= nextRetryTime` with subtraction-safe `(millis() - nextRetryTime)
< 0x80000000UL` in RetryConfig and WiFiRetryConfig; fix boot guard with
static flag instead of raw millis() comparison

H1 (high): Fix DST last-Sunday formula — was using incorrect year-only
heuristic; now derives weekday of the 31st from current day's tm_wday:
`weekdayOf31 = (tm_wday + (31 - day)) % 7`. Verified: March 2026 = 29th ✓

H2 (high): Replace String+= with snprintf+sendContent in handleAPITime,
handleAPIStatus, handleAPIDebug, handleAPIWeather — eliminates permanent
heap fragmentation from JS polling every second

H3 (high): Add volatile to weatherState and ntpState — shared between
ESPAsyncTCP callbacks and main loop; prevents stale register-cached reads

RAM: 37,268 bytes (46%) — reduced from 37,560 due to String elimination

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

firmware/weather_clock/weather.cpp

@@ -156,6 +156,7 @@ void ICACHE_FLASH_ATTR fetchWeatherAsync() {
     weatherRequest.setTimeout(10);  // 10 seconds
     weatherRequest.send();
     weatherState = WEATHER_REQUESTING;
+    weatherRequestStart = millis();
     Serial.println("Weather request sent (non-blocking)");
   } else {
     weatherState = WEATHER_FAILED;