refactor: implement ActivityManager (#1016)

## Summary

Ref comment:
https://github.com/crosspoint-reader/crosspoint-reader/pull/1010#pullrequestreview-3828854640

This PR introduces `ActivityManager`, which mirrors the same concept of
Activity in Android, where an activity represents a single screen of the
UI. The manager is responsible for launching activities, and ensuring
that only one activity is active at a time.

Main differences from Android's ActivityManager:
- No concept of Bundle or Intent extras
- No onPause/onResume, since we don't have a concept of background
activities
- onActivityResult is implemented via a callback instead of a separate
method, for simplicity

## Key changes

- Single `renderTask` shared across all activities
- No more sub-activity, we manage them using a stack; Results can be
passed via `startActivityForResult` and `setResult`
- Activity can call `finish()` to destroy themself, but the actual
deletion will be handled by `ActivityManager` to avoid `delete this`
pattern

As a bonus: the manager will automatically call `requestUpdate()` when
returning from another activity

## Example usage

**BEFORE**:

```cpp
// caller
    enterNewActivity(new WifiSelectionActivity(renderer, mappedInput,
                                               [this](const bool connected) { onWifiSelectionComplete(connected); }));

// subactivity
  onComplete(true); // will eventually call exitActivity(), which deletes the caller instance (dangerous behavior)
``` 

**AFTER**: (mirrors the `startActivityForResult` and `setResult` from
android)

```cpp
// caller
  startActivityForResult(new NetworkModeSelectionActivity(renderer, mappedInput),
                         [this](const ActivityResult& result) { onNetworkModeSelected(result.selectedNetworkMode); });

// subactivity
  ActivityResult result;
  result.isCancelled = false;
  result.selectedNetworkMode = mode;
  setResult(result);
  finish(); // signals to ActivityManager to go back to last activity AFTER this function returns
```

TODO:
- [x] Reconsider if the `Intent` is really necessary or it should be
removed (note: it's inspired by
[Intent](https://developer.android.com/guide/components/intents-common)
from Android API) ==> I decided to keep this pattern fr clarity
- [x] Verify if behavior is still correct (i.e. back from sub-activity)
- [x] Refactor the `ActivityWithSubactivity` to just simple `Activity`
--> We are using a stack for keeping track of sub-activity now
- [x] Use single task for rendering --> avoid allocating 8KB stack per
activity
- [x] Implement the idea of [Activity
result](https://developer.android.com/training/basics/intents/result)
--> Allow sub-activity like Wifi to report back the status (connected,
failed, etc)

---

### 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? **PARTIALLY**, some
repetitive migrations are done by Claude, but I'm the one how ultimately
approve it

---------

Co-authored-by: Zach Nelson <zach@zdnelson.com>

src/activities/reader/TxtReaderActivity.cpp

@@ -23,7 +23,7 @@ constexpr uint8_t CACHE_VERSION = 2;          // Increment when cache format cha
 }  // namespace
 
 void TxtReaderActivity::onEnter() {
-  ActivityWithSubactivity::onEnter();
+  Activity::onEnter();
 
   if (!txt) {
     return;
@@ -61,7 +61,7 @@ void TxtReaderActivity::onEnter() {
 }
 
 void TxtReaderActivity::onExit() {
-  ActivityWithSubactivity::onExit();
+  Activity::onExit();
 
   // Reset orientation back to portrait for the rest of the UI
   renderer.setOrientation(GfxRenderer::Orientation::Portrait);
@@ -74,14 +74,9 @@ void TxtReaderActivity::onExit() {
 }
 
 void TxtReaderActivity::loop() {
-  if (subActivity) {
-    subActivity->loop();
-    return;
-  }
-
   // Long press BACK (1s+) goes to file selection
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= goHomeMs) {
-    onGoBack();
+    activityManager.goToMyLibrary(txt ? txt->getPath() : "");
     return;
   }
 
@@ -325,7 +320,7 @@ bool TxtReaderActivity::loadPageAtOffset(size_t offset, std::vector<std::string>
   return !outLines.empty();
 }
 
-void TxtReaderActivity::render(Activity::RenderLock&&) {
+void TxtReaderActivity::render(RenderLock&&) {
   if (!txt) {
     return;
   }