Rename Screens to Activities and restructure files (#44)

## Summary

* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.

## Additional Context

* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞

src/activities/settings/SettingsActivity.cpp

@@ -0,0 +1,130 @@
+#include "SettingsActivity.h"
+
+#include <GfxRenderer.h>
+
+#include "CrossPointSettings.h"
+#include "config.h"
+
+// Define the static settings list
+
+const SettingInfo SettingsActivity::settingsList[settingsCount] = {
+    {"White Sleep Screen", &CrossPointSettings::whiteSleepScreen},
+    {"Extra Paragraph Spacing", &CrossPointSettings::extraParagraphSpacing}};
+
+void SettingsActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<SettingsActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void SettingsActivity::onEnter() {
+  renderingMutex = xSemaphoreCreateMutex();
+
+  // Reset selection to first item
+  selectedSettingIndex = 0;
+
+  // Trigger first update
+  updateRequired = true;
+
+  xTaskCreate(&SettingsActivity::taskTrampoline, "SettingsActivityTask",
+              2048,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void SettingsActivity::onExit() {
+  // Wait until not rendering to delete task to avoid killing mid-instruction to EPD
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+}
+
+void SettingsActivity::loop() {
+  // Handle actions with early return
+  if (inputManager.wasPressed(InputManager::BTN_CONFIRM)) {
+    toggleCurrentSetting();
+    updateRequired = true;
+    return;
+  }
+
+  if (inputManager.wasPressed(InputManager::BTN_BACK)) {
+    SETTINGS.saveToFile();
+    onGoHome();
+    return;
+  }
+
+  // Handle navigation
+  if (inputManager.wasPressed(InputManager::BTN_UP) || inputManager.wasPressed(InputManager::BTN_LEFT)) {
+    // Move selection up (with wrap-around)
+    selectedSettingIndex = (selectedSettingIndex > 0) ? (selectedSettingIndex - 1) : (settingsCount - 1);
+    updateRequired = true;
+  } else if (inputManager.wasPressed(InputManager::BTN_DOWN) || inputManager.wasPressed(InputManager::BTN_RIGHT)) {
+    // Move selection down (with wrap-around)
+    selectedSettingIndex = (selectedSettingIndex < settingsCount - 1) ? (selectedSettingIndex + 1) : 0;
+    updateRequired = true;
+  }
+}
+
+void SettingsActivity::toggleCurrentSetting() {
+  // Validate index
+  if (selectedSettingIndex < 0 || selectedSettingIndex >= settingsCount) {
+    return;
+  }
+
+  // Toggle the boolean value using the member pointer
+  bool currentValue = SETTINGS.*(settingsList[selectedSettingIndex].valuePtr);
+  SETTINGS.*(settingsList[selectedSettingIndex].valuePtr) = !currentValue;
+
+  // Save settings when they change
+  SETTINGS.saveToFile();
+}
+
+void SettingsActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      render();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void SettingsActivity::render() const {
+  renderer.clearScreen();
+
+  const auto pageWidth = GfxRenderer::getScreenWidth();
+  const auto pageHeight = GfxRenderer::getScreenHeight();
+
+  // Draw header
+  renderer.drawCenteredText(READER_FONT_ID, 10, "Settings", true, BOLD);
+
+  // We always have at least one setting
+
+  // Draw all settings
+  for (int i = 0; i < settingsCount; i++) {
+    const int settingY = 60 + i * 30;  // 30 pixels between settings
+
+    // Draw selection indicator for the selected setting
+    if (i == selectedSettingIndex) {
+      renderer.drawText(UI_FONT_ID, 5, settingY, ">");
+    }
+
+    // Draw setting name and value
+    renderer.drawText(UI_FONT_ID, 20, settingY, settingsList[i].name);
+    bool value = SETTINGS.*(settingsList[i].valuePtr);
+    renderer.drawText(UI_FONT_ID, pageWidth - 80, settingY, value ? "ON" : "OFF");
+  }
+
+  // Draw help text
+  renderer.drawText(SMALL_FONT_ID, 20, pageHeight - 30, "Press OK to toggle, BACK to save & exit");
+
+  // Always use standard refresh for settings screen
+  renderer.displayBuffer();
+}