feat: add silent NTP time sync on boot via saved WiFi credentials

New "Auto Sync on Boot" toggle in Clock Settings. When enabled, a
background FreeRTOS task scans for saved WiFi networks at boot,
connects, syncs time via NTP, then tears down WiFi — all without
blocking boot or requiring user interaction. If no saved network is
found after two scan attempts (with a 3-second retry gap), it bails
silently.

Conflict guards (BootNtpSync::cancel()) added to all WiFi-using
activities so the background task cleans up before any user-initiated
WiFi flow. Also fixes clock not appearing in the header until a button
press by detecting the invalid→valid time transition after NTP sync.

Made-with: Cursor

src/SettingsList.h

@@ -91,6 +91,8 @@ inline std::vector<SettingInfo> getSettingsList() {
                         {StrId::STR_TZ_UTC, StrId::STR_TZ_EASTERN, StrId::STR_TZ_CENTRAL, StrId::STR_TZ_MOUNTAIN,
                          StrId::STR_TZ_PACIFIC, StrId::STR_TZ_ALASKA, StrId::STR_TZ_HAWAII, StrId::STR_TZ_CUSTOM},
                         "timezone", StrId::STR_CAT_CLOCK),
+      SettingInfo::Toggle(StrId::STR_AUTO_NTP_SYNC, &CrossPointSettings::autoNtpSync, "autoNtpSync",
+                          StrId::STR_CAT_CLOCK),
 
       // --- Reader ---
       SettingInfo::DynamicEnum(