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/globals.h

@@ -29,9 +29,9 @@ extern NTPClient timeClient;
 extern ESP8266WebServer server;
 extern ESP8266HTTPUpdateServer httpUpdater;
 
-// State machines
-extern WeatherState weatherState;
-extern NTPState ntpState;
+// State machines — volatile: written from ESPAsyncTCP callbacks, read in main loop
+extern volatile WeatherState weatherState;
+extern volatile NTPState ntpState;
 extern WiFiConnectionState wifiConnState;
 
 // Retry configurations
@@ -71,6 +71,7 @@ extern SunTimes sunTimes;
 extern uint8_t displayMode;
 extern unsigned long lastModeSwitch;
 extern unsigned long lastWeatherUpdate;
+extern unsigned long weatherRequestStart;
 
 // Dissolve transition state
 extern bool inTransition;