Redesign approach for CPU usage tracking

The previous implementation maintained three exponential moving averages
(1/5/15-minute) of CPU busy-fraction, sampled every 5 seconds. This had
two problems:

1. Wrong semantics: the metric was labelled "load_avg" (a Unix concept
   measuring run-queue depth), but the tracker actually measures CPU
   busy-time as a fraction of FreeRTOS ticks — i.e. utilization, not
   load.  The name was misleading.

2. Poor responsiveness: the 1-minute EMA (DECAY = exp(-5/60) ≈ 0.920)
   has a ~60-second time constant.  On a live web panel, this made the
   metric appear frozen, especially during short bursts of activity
   (e.g. an HTTP request from the panel itself).

This commit replaces the EMA with a 64-sample simple moving average
(SMA) over a compact uint8_t circular buffer. The sample interval is
60 s / 64 = 937,500 µs, so the window covers exactly 60 seconds:

Algorithm:
- Every 937,500 µs (= 60 s / 64), the FreeRTOS tick delta (busy vs
  idle ticks on core 0) is computed and stored as a uint8_t
  (0–255 = 0–100% utilization).
- The circular buffer holds 64 samples, spanning exactly 60 seconds
  (64 × 937,500 µs = 60,000,000 µs). The timer interval is derived
  directly from the window size: 60 000 000 / SMA_WINDOW µs.
- The index wraps with a bitwise AND (& 63) instead of modulo, since
  64 is a power of 2.
- The running sum is a uint16_t (max 64 × 255 = 16 320, fits easily).
- The result _sma_avg is a volatile float written atomically by the
  esp_timer task (core 0) and read from the main loop (core 1).
  No spinlock is needed: on Xtensa LX7, a 32-bit aligned float store
  is a single instruction.

The 60-second window smooths out short bursts (e.g. WiFi/HTTP spikes)
while reacting to sustained load changes within ~10–15 seconds.

Naming:
- The public API is now getCore0Util() returning a float in [0.0, 1.0].
  The name explicitly identifies which core is measured (core 0, which
  runs WiFi, MQTT, HTTP, and LwIP — see task_pinning.c).
- JSON keys: "load_avg" (array) → "core0_util" (scalar float, percent)
- HistorySample field: load_avg1_pct → core0_util_pct
- StatsHistory series key: "cpu_load" → "core0_util"
- Web panel label: "Load Avg" → "Core0 Util"

The history snapshot (once per minute) reads the same _sma_avg value,
which at that point represents the rolling average of the last 60
seconds — exactly one history interval.

arch/esp32/CPUUsageTracker.h

@@ -11,14 +11,10 @@ class CPUUsageTracker {
 public:
   void begin();
 
-  float getLoadAvg1()   const { return _avg1; }
-  float getLoadAvg5()   const { return _avg5; }
-  float getLoadAvg15()  const { return _avg15; }
+  float getCore0Util() const { return _sma_avg; }
 
 private:
-  static constexpr float DECAY1  = 0.920044415f;  // exp(-5/60)
-  static constexpr float DECAY5  = 0.983471454f;  // exp(-5/300)
-  static constexpr float DECAY15 = 0.994459848f;  // exp(-5/900)
+  static constexpr uint8_t SMA_WINDOW = 64;  // power of 2 — enables & mask
 
   static volatile uint32_t s_idle_ticks;
   static volatile uint32_t s_busy_ticks;
@@ -27,11 +23,15 @@ private:
   static void IRAM_ATTR s_tick_hook();
   static void           s_sample_cb(void* arg);
 
-  uint32_t _last_idle   = 0;
-  uint32_t _last_busy   = 0;
-  float _avg1           = 0.0f;
-  float _avg5           = 0.0f;
-  float _avg15          = 0.0f;
+  uint32_t _last_idle = 0;
+  uint32_t _last_busy = 0;
+
+  uint8_t  _sma_buf[SMA_WINDOW] = {};
+  uint16_t _sma_sum = 0;
+  uint8_t  _sma_idx = 0;
+
+  volatile float _sma_avg = 0.0f;
+
   esp_timer_handle_t _timer = nullptr;
 
   void _onSample();