Web: fix bogus packets/error_rate delta at SD archive restore boundary

restoreSummaryLog() seeds the ring buffer with one sample from the
previous session (summary.latest).  The next live sample has reset
counters and uptime starting from zero, so the delta across that
boundary is meaningless.

For the packets series this produced a zero point (same symptom as the
first-sample zero fixed in 1018404b, but at the restore boundary instead).
For error_rate it produced an inflated value because the counter difference
wraps around.

Fix by skipping any sample whose uptime_secs does not exceed the
previous sample's uptime_secs.  In buildPointValue this returns false;
in the buildSeriesJson packets loop it skips emission and does not
advance previous.

Also: move have_previous = false into the else branch (the packets
branch does not use it), drop the now-redundant >= guards in favour of
direct uint32_t subtraction, and replace a C-style cast with
static_cast<int> for consistency.

src/helpers/StatsHistory.cpp

@@ -228,16 +228,16 @@ bool buildPointValue(const HistorySample& sample, const HistorySample* previous,
     return true;
   }
   if (strcmp(series, "packets") == 0) {
-    if (previous == nullptr) {
+    if (previous == nullptr || sample.uptime_secs < previous->uptime_secs) {
       return false;
     }
-    const uint32_t curr_total = sample.packets_sent + sample.packets_recv;
-    const uint32_t prev_total = previous->packets_sent + previous->packets_recv;
-    value = static_cast<int>(curr_total >= prev_total ? (curr_total - prev_total) : 0);
+    const uint32_t d_recv = sample.packets_recv - previous->packets_recv;
+    const uint32_t d_sent = sample.packets_sent - previous->packets_sent;
+    value = static_cast<int>(d_recv + d_sent);
     return true;
   }
   if (strcmp(series, "error_rate") == 0) {
-    if (previous == nullptr) {
+    if (previous == nullptr || sample.uptime_secs < previous->uptime_secs) {
       return false;
     }
     const uint32_t d_errors = sample.recv_errors - previous->recv_errors;
@@ -246,7 +246,7 @@ bool buildPointValue(const HistorySample& sample, const HistorySample* previous,
     if (total == 0) {
       return false;
     }
-    value = (int)((d_errors * 1000u) / total);
+    value = static_cast<int>((d_errors * 1000u) / total);
     return true;
   }
   if (strcmp(series, "voltage") == 0) {
@@ -1342,30 +1342,28 @@ bool StatsHistory::buildSeriesJson(const char* series, char* buffer, size_t buff
   offset = static_cast<size_t>(header_written);
   emitted = 0;
   size_t valid_emitted = 0;
-  have_previous = false;
   if (strcmp(series, "packets") == 0) {
     if (_sample_count >= 1 && getSampleFromOldest(0, previous)) {
       for (size_t i = step; i < _sample_count && emitted < points; i += step, ++emitted) {
-        if (!getSampleFromOldest(i, sample)) {
-          break;
-        }
-        const int rx = sample.packets_recv >= previous.packets_recv
-          ? (int)(sample.packets_recv - previous.packets_recv) : 0;
-        const int tx = sample.packets_sent >= previous.packets_sent
-          ? (int)(sample.packets_sent - previous.packets_sent) : 0;
-        offset += snprintf(&buffer[offset], buffer_size - offset,
-                          "%s[%lu,%d,%d]",
-                          valid_emitted == 0 ? "" : ",",
-                          static_cast<unsigned long>(sample.uptime_secs),
-                          rx, tx);
-        valid_emitted++;
-        if (offset + 40 >= buffer_size) {
-          break;
+        getSampleFromOldest(i, sample);
+        if (sample.uptime_secs > previous.uptime_secs) {
+          const uint32_t rx = sample.packets_recv - previous.packets_recv;
+          const uint32_t tx = sample.packets_sent - previous.packets_sent;
+          offset += snprintf(&buffer[offset], buffer_size - offset,
+                             "%s[%lu,%d,%d]",
+                             valid_emitted == 0 ? "" : ",",
+                             static_cast<unsigned long>(sample.uptime_secs),
+                             static_cast<int>(rx), static_cast<int>(tx));
+          valid_emitted++;
+          if (offset + 40 >= buffer_size) {
+            break;
+          }
         }
         previous = sample;
       }
     }
   } else {
+    have_previous = false;
     for (size_t i = 0; i < _sample_count && emitted < points; i += step, ++emitted) {
       if (!getSampleFromOldest(i, sample)) {
         break;