refactor: move render() to Activity super class, use freeRTOS notification (#774)

## Summary

Currently, each activity has to manage their own `displayTaskLoop` which
adds redundant boilerplate code. The loop is a wait loop which is also
not the best practice, as the `updateRequested` boolean is not protected
by a mutex.

In this PR:
- Move `displayTaskLoop` to the super `Activity` class
- Replace `updateRequested` with freeRTOS's [direct to task
notification](https://www.freertos.org/Documentation/02-Kernel/02-Kernel-features/03-Direct-to-task-notifications/01-Task-notifications)
- For `ActivityWithSubactivity`, whenever a sub-activity is present, the
parent's `render()` automatically goes inactive

With this change, activities now only need to expose `render()`
function, and anywhere in the code base can call `requestUpdate()` to
request a new rendering pass.

## Additional Context

In theory, this change may also make the battery life a bit better,
since one wait loop is removed. Although the equipment in my home lab
wasn't been able to verify it (the electric current is too noisy and
small). Would appreciate if anyone has any insights on this subject.

Update: I managed to hack [a small piece of
code](https://github.com/ngxson/crosspoint-reader/tree/xsn/measure_cpu_usage)
that allow tracking CPU idle time.

The CPU load does decrease a bit (1.47% down to 1.39%), which make
sense, because the display task is now sleeping most of the time unless
notified. This should translate to a slightly increase in battery life
in the long run.

```
PR:
[40012] [MEM] Free: 185856 bytes, Total: 231004 bytes, Min Free: 123316 bytes
[40012] [IDLE] Idle time: 98.61% (CPU load: 1.39%)
[50017] [MEM] Free: 185856 bytes, Total: 231004 bytes, Min Free: 123316 bytes
[50017] [IDLE] Idle time: 98.61% (CPU load: 1.39%)
[60022] [MEM] Free: 185856 bytes, Total: 231004 bytes, Min Free: 123316 bytes
[60022] [IDLE] Idle time: 98.61% (CPU load: 1.39%)

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%)
```

---

### 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**


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Refactor**
* Streamlined rendering architecture by consolidating update mechanisms
across all activities, improving efficiency and consistency.
* Modernized synchronization patterns for display updates to ensure
reliable, conflict-free rendering.

* **Bug Fixes**
* Enhanced rendering stability through improved locking mechanisms and
explicit update requests.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: znelson <znelson@users.noreply.github.com>

src/activities/reader/TxtReaderActivity.cpp

@@ -23,11 +23,6 @@ constexpr uint32_t CACHE_MAGIC = 0x54585449;  // "TXTI"
 constexpr uint8_t CACHE_VERSION = 2;          // Increment when cache format changes
 }  // namespace
 
-void TxtReaderActivity::taskTrampoline(void* param) {
-  auto* self = static_cast<TxtReaderActivity*>(param);
-  self->displayTaskLoop();
-}
-
 void TxtReaderActivity::onEnter() {
   ActivityWithSubactivity::onEnter();
 
@@ -53,8 +48,6 @@ void TxtReaderActivity::onEnter() {
       break;
   }
 
-  renderingMutex = xSemaphoreCreateMutex();
-
   txt->setupCacheDir();
 
   // Save current txt as last opened file and add to recent books
@@ -65,14 +58,7 @@ void TxtReaderActivity::onEnter() {
   RECENT_BOOKS.addBook(filePath, fileName, "", "");
 
   // Trigger first update
-  updateRequired = true;
-
-  xTaskCreate(&TxtReaderActivity::taskTrampoline, "TxtReaderActivityTask",
-              6144,               // Stack size
-              this,               // Parameters
-              1,                  // Priority
-              &displayTaskHandle  // Task handle
-  );
+  requestUpdate();
 }
 
 void TxtReaderActivity::onExit() {
@@ -81,14 +67,6 @@ void TxtReaderActivity::onExit() {
   // Reset orientation back to portrait for the rest of the UI
   renderer.setOrientation(GfxRenderer::Orientation::Portrait);
 
-  // Wait until not rendering to delete task
-  xSemaphoreTake(renderingMutex, portMAX_DELAY);
-  if (displayTaskHandle) {
-    vTaskDelete(displayTaskHandle);
-    displayTaskHandle = nullptr;
-  }
-  vSemaphoreDelete(renderingMutex);
-  renderingMutex = nullptr;
   pageOffsets.clear();
   currentPageLines.clear();
   APP_STATE.readerActivityLoadCount = 0;
@@ -134,22 +112,10 @@ void TxtReaderActivity::loop() {
 
   if (prevTriggered && currentPage > 0) {
     currentPage--;
-    updateRequired = true;
+    requestUpdate();
   } else if (nextTriggered && currentPage < totalPages - 1) {
     currentPage++;
-    updateRequired = true;
-  }
-}
-
-void TxtReaderActivity::displayTaskLoop() {
-  while (true) {
-    if (updateRequired) {
-      updateRequired = false;
-      xSemaphoreTake(renderingMutex, portMAX_DELAY);
-      renderScreen();
-      xSemaphoreGive(renderingMutex);
-    }
-    vTaskDelay(10 / portTICK_PERIOD_MS);
+    requestUpdate();
   }
 }
 
@@ -372,7 +338,7 @@ bool TxtReaderActivity::loadPageAtOffset(size_t offset, std::vector<std::string>
   return !outLines.empty();
 }
 
-void TxtReaderActivity::renderScreen() {
+void TxtReaderActivity::render(Activity::RenderLock&&) {
   if (!txt) {
     return;
   }