feat: more power saving on idle (#801)

## Summary

This PR extends the delay in main loop from 10ms to 50ms after the
device is idle for a while. This translates to extended battery life in
a longer period (see testing section above), while not hurting too much
the user experience.

With the help from [this
patch](https://github.com/ngxson/crosspoint-reader/tree/xsn/measure_cpu_usage),
I was able to measure the CPU usage on idle:

```
PR:
[20017] [MEM] Free: 150188 bytes, Total: 232092 bytes, Min Free: 150092 bytes
[20017] [IDLE] Idle time: 99.62% (CPU load: 0.38%)
[30042] [MEM] Free: 150188 bytes, Total: 232092 bytes, Min Free: 150092 bytes
[30042] [IDLE] Idle time: 99.63% (CPU load: 0.37%)
[40067] [MEM] Free: 150188 bytes, Total: 232092 bytes, Min Free: 150092 bytes
[40067] [IDLE] Idle time: 99.62% (CPU load: 0.38%)

master:
[20012] [MEM] Free: 195016 bytes, Total: 231532 bytes, Min Free: 132460 bytes
[20012] [IDLE] Idle time: 98.53% (CPU load: 1.47%)
[30017] [MEM] Free: 195016 bytes, Total: 231532 bytes, Min Free: 132460 bytes
[30017] [IDLE] Idle time: 98.53% (CPU load: 1.47%)
[40022] [MEM] Free: 195016 bytes, Total: 231532 bytes, Min Free: 132460 bytes
[40022] [IDLE] Idle time: 98.53% (CPU load: 1.47%)
```

While this is a x3.8 reduce in CPU usage, it doesn't translate to the
same amount of battery life extension in real life. The reasons are:
1. The CPU is not shut down completely
2. freeRTOS tick is still running (however, I planned to experiment with
tickless functionality)
3. Current leakage to other components, for example: voltage dividers,
eink screen, SD card, etc

A note on
[light-sleep](https://docs.espressif.com/projects/esp-idf/en/stable/esp32c3/api-reference/system/sleep_modes.html)
functionality: it is not possible in our use case because:
- Light-sleep for 50ms introduce too much overhead on wake up, it has
negative effect on battery life
- Light-sleep for longer period doesn't work because the ADC GPIO
buttons cannot be used as wake up source

## Testing (duration = 6 hrs)

To test this, I patched the `CrossPointSettings::getSleepTimeoutMs()` to
always returns a timeout of 6 hrs. This allow me to leave the device
idle for 6 hrs straight.

- On master branch, 6 hrs costs 26% battery life (100% --> 74%), meaning
battery life is ~23 hrs
- With this PR, 6 hrs costs 20% battery life (100% --> 80%), meaning
battery life is ~30 hrs

So in theory, this extends the battery by about 7 hrs. Even with some
error margin added, I think 3 hrs increase is possible with a normal
usage setup (i.e. only read ebooks, no wifi)

## Additional Context

Would appreciate if someone can test this with an oscilloscope.

---

### 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

@@ -410,6 +410,13 @@ void loop() {
   if (currentActivity && currentActivity->skipLoopDelay()) {
     yield();  // Give FreeRTOS a chance to run tasks, but return immediately
   } else {
-    delay(10);  // Normal delay when no activity requires fast response
+    static constexpr unsigned long IDLE_POWER_SAVING_MS = 3000;  // 3 seconds
+    if (millis() - lastActivityTime >= IDLE_POWER_SAVING_MS) {
+      // If we've been inactive for a while, increase the delay to save power
+      delay(50);
+    } else {
+      // Short delay to prevent tight loop while still being responsive
+      delay(10);
+    }
   }
 }