Stats: add CPU load trend series to history and web panel

Wire the 1-minute CPU load average into the stats history pipeline:

- Rename HistorySample::reserved to load_avg1_pct (uint8_t, 0-100);
  struct size unchanged.
- Populate load_avg1_pct in updateStatsHistory on ESP32 from
  _cpu_tracker.getLoadAvg1() * 100.
- Register "cpu_load" series in buildPointValue / seriesTitle /
  seriesUnit so buildSeriesJson serves it via
  /api/stats?series=cpu_load.
- Persist load_avg1_pct as a 29th CSV column in flushSummaryLog.
  parseSummaryLine accepts both 28-column (old, load_avg1_pct=0)
  and 29-column (new) formats for backward compatibility.
- Add "cpu_load" to the web panel trend card order with orange
  sparkline color (#e07b39), "N %" hover formatting, and a Y-axis
  floored at 0 with a 10% minimum ceiling.

examples/simple_repeater/MyMesh.cpp

@@ -1941,6 +1941,7 @@ void MyMesh::updateStatsHistory(unsigned long now_ms) {
       sample.heap_min = ESP.getMinFreeHeap();
       sample.psram_free = ESP.getFreePsram();
       sample.psram_min = ESP.getMinFreePsram();
+      sample.load_avg1_pct = (uint8_t)(_cpu_tracker.getLoadAvg1() * 100.0f + 0.5f);
 #endif
       if (board.isExternalPowered()) sample.flags |= HISTORY_FLAG_EXTERNAL_POWER;
       if (board.isCharging()) sample.flags |= HISTORY_FLAG_CHARGING;