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/main.cpp

@@ -32,6 +32,7 @@
 #include "activities/util/FullScreenMessageActivity.h"
 #include "components/UITheme.h"
 #include "fontIds.h"
+#include "util/BootNtpSync.h"
 #include "util/ButtonNavigator.h"
 
 HalDisplay display;
@@ -342,6 +343,7 @@ void setup() {
 
   I18N.loadSettings();
   KOREADER_STORE.loadFromFile();
+  BootNtpSync::start();
   UITheme::getInstance().reload();
   ButtonNavigator::setMappedInputManager(mappedInputManager);
 
@@ -459,14 +461,23 @@ void loop() {
   // Refresh screen when the displayed minute changes (clock in header)
   if (SETTINGS.clockFormat != CrossPointSettings::CLOCK_OFF && currentActivity) {
     static int lastRenderedMinute = -1;
+    static bool sawInvalidTime = false;
     time_t now = time(nullptr);
     struct tm* t = localtime(&now);
     if (t != nullptr && t->tm_year > 100) {
       const int currentMinute = t->tm_hour * 60 + t->tm_min;
-      if (lastRenderedMinute >= 0 && currentMinute != lastRenderedMinute) {
+      if (lastRenderedMinute < 0) {
+        lastRenderedMinute = currentMinute;
+        if (sawInvalidTime) {
+          // Time just became valid (e.g. background NTP sync completed)
+          currentActivity->requestUpdate();
+        }
+      } else if (currentMinute != lastRenderedMinute) {
         currentActivity->requestUpdate();
+        lastRenderedMinute = currentMinute;
       }
-      lastRenderedMinute = currentMinute;
+    } else {
+      sawInvalidTime = true;
     }
   }