Various improvements (#461)

* **New Features**
  * Time and delay utilities added (ticks, ms, µs); SD card now uses an expansion-header CS pin; HTTP downloads warn when run on the GUI task and yield to avoid blocking.

* **Bug Fixes / Reliability**
  * Many hard-crash paths converted to guarded checks to reduce abrupt termination and improve stability.

* **Tests**
  * Unit tests added to validate time and delay accuracy.

* **Chores**
  * License header and build/macro updates.

TactilityKernel/Include/Tactility/Time.h

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "Tactility/FreeRTOS/task.h"
+
+#ifdef ESP_PLATFORM
+#include <esp_timer.h>
+#else
+#include <sys/time.h>
+#include <time.h>
+#endif
+
+// Projects that include this header must align with Tactility's frequency (e.g. apps)
+static_assert(configTICK_RATE_HZ == 1000);
+
+static inline uint32_t get_tick_frequency() {
+    return configTICK_RATE_HZ;
+}
+
+/** @return the amount of ticks that have passed since FreeRTOS' main task started */
+static inline TickType_t get_ticks() {
+    if (xPortInIsrContext() == pdTRUE) {
+        return xTaskGetTickCountFromISR();
+    } else {
+        return xTaskGetTickCount();
+    }
+}
+
+/** @return the milliseconds that have passed since FreeRTOS' main task started */
+static inline size_t get_millis() {
+    return get_ticks() * portTICK_PERIOD_MS;
+}
+
+/** @return the frequency at which the kernel task schedulers operate */
+uint32_t kernel_get_tick_frequency();
+
+/** @return the microseconds that have passed since boot */
+static inline int64_t get_micros_since_boot() {
+#ifdef ESP_PLATFORM
+    return esp_timer_get_time();
+#else
+    timespec ts;
+    if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
+        return (static_cast<int64_t>(ts.tv_sec) * 1000000LL) + (ts.tv_nsec / 1000);
+    }
+    timeval tv;
+    gettimeofday(&tv, nullptr);
+    return (static_cast<int64_t>(tv.tv_sec) * 1000000LL) + tv.tv_usec;
+#endif
+}
+
+/** Convert seconds to ticks */
+static inline TickType_t seconds_to_ticks(uint32_t seconds) {
+    return static_cast<uint64_t>(seconds) * 1000U / portTICK_PERIOD_MS;
+}
+
+/** Convert milliseconds to ticks */
+static inline TickType_t millis_to_ticks(uint32_t milliSeconds) {
+#if configTICK_RATE_HZ == 1000
+    return static_cast<TickType_t>(milliSeconds);
+#else
+    return static_cast<TickType_t>(((float)configTICK_RATE_HZ) / 1000.0f * (float)milliSeconds);
+#endif
+}