fix: post-review issues + close M2/M3/M4 from internal backlog (v1.9.9)

From Sonnet pre-release code review (3 real issues):
- F1 (high): factory reset atomicity — save cleared creds BEFORE zeroing reset
  counter, so a power loss between commits still results in WiFiManager AP boot
  instead of inconsistent "counter=0 + stale creds" state
- F2: isValidSSID now allows single-char SSIDs (per 802.11 spec)
- F3: ntp_interval changes now trigger needsRestart (NTPClient constructed once
  in setup() with this value, doesn't pick up runtime changes)

From internal backlog:
- M2: setupWiFi Try-2 now supports open networks (no password) — fixed the
  &&-condition that required both ssid and password
- M3: removed dead NTP_WAITING/NTP_SUCCESS/NTP_FAILED enum values
- M4: randomSeed() with ESP.getChipId() ^ micros() — dissolve pattern varies

Tested on hardware: 72/73 tests pass (single failure is expected — ntp_interval
fuzz cases now trigger restart due to F3, second case lands in reboot window).

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

firmware/weather_clock/wifi_manager.cpp

@@ -105,10 +105,14 @@ void ICACHE_FLASH_ATTR setupWiFi() {
     }
   }
 
-  // Try 2: If we have EEPROM credentials, try those
-  if (strlen(config.ssid) > 0 && strlen(config.password) > 0) {
+  // Try 2: If we have EEPROM credentials, try those (M2: support open networks)
+  if (strlen(config.ssid) > 0) {
     Serial.println("\nTrying EEPROM credentials...");
-    WiFi.begin(config.ssid, config.password);
+    if (strlen(config.password) > 0) {
+      WiFi.begin(config.ssid, config.password);
+    } else {
+      WiFi.begin(config.ssid);  // open network — no password
+    }
 
     int attempts = 0;
     while (WiFi.status() != WL_CONNECTED && attempts < 20) {