feat: lower CPU freq on idle, add HalPowerManager (#852)

## Summary

Continue my experiment from
https://github.com/crosspoint-reader/crosspoint-reader/pull/801

This PR add the ability to lower the CPU frequency on extended idle
period (currently set to 3 seconds). By default, the esp32c3 CPU is set
to 160MHz, and now on idle, we can reduce it to just 10MHz.

Note that while this functionality is already provided by [esp power
management](https://docs.espressif.com/projects/esp-idf/en/v4.3/esp32c3/api-reference/system/power_management.html),
the current Arduino build lacks of this, and enabling it is just too
complicated (not worth the effort compared to this PR)

Update: more info in
https://github.com/crosspoint-reader/crosspoint-reader/pull/852#issuecomment-3904562827

## Testing

Pre-condition for each test case: the battery is charged to 100%, and is
left plugged in after fully charged for an extra 1 hour.

The table below shows how much battery is **used** for a given duration:

| case / duration | 6 hrs | 12 hrs |
| --- | --- | --- |
| `delay(10)` | 26% | 48% |
| `delay(50)`, PR
https://github.com/crosspoint-reader/crosspoint-reader/pull/801 | 20% |
Not tested |
| `delay(50)` + low CPU freq (This PR) | Not tested | 25% |
| `delay(10)` + low CPU freq (1) | Not tested | Not tested |

(1) I decided not to test this case because it may not make sense. The
problem is that CPU frequency vs power consumption do not follow a
linear relationship, see
[this](https://www.arrow.com/en/research-and-events/articles/esp32-power-consumption-can-be-reduced-with-sleep-modes)
as an example. So, tight loop (10ms) + lower CPU freq significantly
impact battery life, because the active CPU time is now much higher
compared to the wall time.

**So in conclusion, this PR improves ~150% to ~200% battery use time per
charge.**

The projected battery life is now: ~36-48 hrs of reading time (normal
reading, no wifi)

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? **NO**

src/main.cpp

@@ -3,6 +3,7 @@
 #include <GfxRenderer.h>
 #include <HalDisplay.h>
 #include <HalGPIO.h>
+#include <HalPowerManager.h>
 #include <HalStorage.h>
 #include <I18n.h>
 #include <Logging.h>
@@ -183,7 +184,7 @@ void verifyPowerButtonDuration() {
   if (abort) {
     // Button released too early. Returning to sleep.
     // IMPORTANT: Re-arm the wakeup trigger before sleeping again
-    gpio.startDeepSleep();
+    powerManager.startDeepSleep(gpio);
   }
 }
 
@@ -206,7 +207,7 @@ void enterDeepSleep() {
   LOG_DBG("MAIN", "Power button press calibration value: %lu ms", t2 - t1);
   LOG_DBG("MAIN", "Entering deep sleep");
 
-  gpio.startDeepSleep();
+  powerManager.startDeepSleep(gpio);
 }
 
 void onGoHome();
@@ -283,6 +284,7 @@ void setup() {
   t1 = millis();
 
   gpio.begin();
+  powerManager.begin();
 
   // Only start serial if USB connected
   if (gpio.isUsbConnected()) {
@@ -319,7 +321,7 @@ void setup() {
     case HalGPIO::WakeupReason::AfterUSBPower:
       // If USB power caused a cold boot, go back to sleep
       LOG_DBG("MAIN", "Wakeup reason: After USB Power");
-      gpio.startDeepSleep();
+      powerManager.startDeepSleep(gpio);
       break;
     case HalGPIO::WakeupReason::AfterFlash:
       // After flashing, just proceed to boot
@@ -391,7 +393,8 @@ void loop() {
   // Check for any user activity (button press or release) or active background work
   static unsigned long lastActivityTime = millis();
   if (gpio.wasAnyPressed() || gpio.wasAnyReleased() || (currentActivity && currentActivity->preventAutoSleep())) {
-    lastActivityTime = millis();  // Reset inactivity timer
+    lastActivityTime = millis();         // Reset inactivity timer
+    powerManager.setPowerSaving(false);  // Restore normal CPU frequency on user activity
   }
 
   const unsigned long sleepTimeoutMs = SETTINGS.getSleepTimeoutMs();
@@ -426,11 +429,12 @@ void loop() {
   // When an activity requests skip loop delay (e.g., webserver running), use yield() for faster response
   // Otherwise, use longer delay to save power
   if (currentActivity && currentActivity->skipLoopDelay()) {
-    yield();  // Give FreeRTOS a chance to run tasks, but return immediately
+    powerManager.setPowerSaving(false);  // Make sure we're at full performance when skipLoopDelay is requested
+    yield();                             // Give FreeRTOS a chance to run tasks, but return immediately
   } else {
-    static constexpr unsigned long IDLE_POWER_SAVING_MS = 3000;  // 3 seconds
-    if (millis() - lastActivityTime >= IDLE_POWER_SAVING_MS) {
+    if (millis() - lastActivityTime >= HalPowerManager::IDLE_POWER_SAVING_MS) {
       // If we've been inactive for a while, increase the delay to save power
+      powerManager.setPowerSaving(true);  // Lower CPU frequency after extended inactivity
       delay(50);
     } else {
       // Short delay to prevent tight loop while still being responsive