feat: persist pressure history to LittleFS across reboots

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/WeatherPredictor.ino

@@ -13,6 +13,7 @@ AppState g_state;
 
 static unsigned long tSample  = 0;
 static unsigned long tHistory = 0;
+static unsigned long tFlush   = 0;
 
 static void sampleNow() {
   g_state.now    = rtcNow();
@@ -47,12 +48,14 @@ void setup() {
   }
   settingsBegin();
   historyBegin();
+  historyLoad();   // restore prior samples (empty on first ever boot)
+  Serial.printf("history restored: %d samples\n", historyCount());
 
   // Seed first sample immediately.
   sampleNow();
   historyAdd(rtcEpoch(), g_state.mslHpa, g_state.tempC);
   displayRender(g_state);
-  tSample = tHistory = millis();
+  tSample = tHistory = tFlush = millis();
 }
 
 void loop() {
@@ -68,4 +71,10 @@ void loop() {
     tHistory = now;
     historyAdd(rtcEpoch(), g_state.mslHpa, g_state.tempC);
   }
+
+  if (now - tFlush >= FLUSH_INTERVAL_MS) {
+    tFlush = now;
+    historySave();
+    Serial.printf("history flushed: %d samples\n", historyCount());
+  }
 }