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/web_server.cpp

@@ -294,7 +294,8 @@ static bool isValidSSID(const String& s) {
     if (c < 0x20 || c > 0x7E) return false;  // non-printable
     if (c != first) allSame = false;
   }
-  return !allSame;  // reject "AAAAA...", "BBBBB...", etc.
+  // Single-char SSIDs ("A") are valid per 802.11. Only reject all-same for length>1.
+  return n == 1 || !allSame;
 }
 
 void ICACHE_FLASH_ATTR handleConfigSave() {
@@ -358,7 +359,9 @@ void ICACHE_FLASH_ATTR handleConfigSave() {
   }
   if (server.hasArg("ntp_interval")) {
     long ni = server.arg("ntp_interval").toInt();
-    config.ntp_interval = constrain(ni, 60L, 86400L);
+    unsigned long newInterval = constrain(ni, 60L, 86400L);
+    if (newInterval != config.ntp_interval) needsRestart = true;  // NTPClient constructed at boot with this
+    config.ntp_interval = newInterval;
   }
   if (server.hasArg("ntp_server")) {
     String n = server.arg("ntp_server");