Stopwatch counts in real time from a start date, kept over a restart

The stopwatch counted with millis(), so it started at 0 after every
restart and would overflow after 49 days. It now keeps the real start
time (UTC ms) and the time counted before a pause, stored in NVS
(system/stopwatch_store) on every change, so it keeps counting while
the display is off or restarting. Stop pauses, Start continues, and the
Stopwatch tab can set the start to a date and time (local, summer time
by mktime) to count from, e.g. days since an event. Start, stop and set
need the synced time.

New page formats: 12d 4:05:09 and 3w 2d.

Version 2.19.0, tested on the display over OTA (set start, future and
invalid dates refused, pause, restart).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/display/page_resources.cpp

@@ -1,6 +1,7 @@
 #include "display/page_resources.h"
 
 #include <Arduino.h>
+#include <sys/time.h>
 
 #include "display/bitmap_store.h"
 #include "display/font_store.h"
@@ -32,9 +33,16 @@ const char *StoredPageResources::haValue(const PageElement &element)
 	return haApiValue(element.entity, element.attribute);
 }
 
-uint32_t StoredPageResources::stopwatchMs()
+uint64_t StoredPageResources::stopwatchMs()
 {
-	return _stopwatch.elapsed(millis());
+	return _stopwatch.elapsed(epochMs());
+}
+
+int64_t epochMs()
+{
+	struct timeval tv;
+	gettimeofday(&tv, nullptr);
+	return (int64_t)tv.tv_sec * 1000 + tv.tv_usec / 1000;
 }
 
 bool StoredPageResources::stopwatchRunning()