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**

lib/hal/HalPowerManager.h

@@ -0,0 +1,56 @@
+#pragma once
+
+#include <Arduino.h>
+#include <InputManager.h>
+#include <Logging.h>
+#include <freertos/semphr.h>
+
+#include <cassert>
+
+#include "HalGPIO.h"
+
+class HalPowerManager;
+extern HalPowerManager powerManager;  // Singleton
+
+class HalPowerManager {
+  int normalFreq = 0;  // MHz
+  bool isLowPower = false;
+
+  enum LockMode { None, NormalSpeed };
+  LockMode currentLockMode = None;
+  SemaphoreHandle_t modeMutex = nullptr;  // Protect access to currentLockMode
+
+ public:
+  static constexpr int LOW_POWER_FREQ = 10;                    // MHz
+  static constexpr unsigned long IDLE_POWER_SAVING_MS = 3000;  // ms
+
+  void begin();
+
+  // Control CPU frequency for power saving
+  void setPowerSaving(bool enabled);
+
+  // Setup wake up GPIO and enter deep sleep
+  // Should be called inside main loop() to handle the currentLockMode
+  void startDeepSleep(HalGPIO& gpio) const;
+
+  // Get battery percentage (range 0-100)
+  int getBatteryPercentage() const;
+
+  // RAII helper class to manage power saving locks
+  // Usage: create an instance of Lock in a scope to disable power saving, for example when running a task that needs
+  // full performance. When the Lock instance is destroyed (goes out of scope), power saving will be re-enabled.
+  class Lock {
+    friend class HalPowerManager;
+    bool valid = false;
+
+   public:
+    explicit Lock();
+    ~Lock();
+
+    // Non-copyable and non-movable
+    Lock(const Lock&) = delete;
+    Lock& operator=(const Lock&) = delete;
+    Lock(Lock&&) = delete;
+    Lock& operator=(Lock&&) = delete;
+  };
+};