feat: Go To Position for epubs (#666)

## Summary

* Adds Go To % action in Epub Reader menu with slider style percent
selector

<img width="860" height="1147" alt="image"
src="https://github.com/user-attachments/assets/a38ecc71-429e-40e8-94ac-37fb1509dbd9"
/>

---

### 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 >**_

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/activities/reader/EpubReaderPercentSelectionActivity.cpp

@@ -0,0 +1,139 @@
+#include "EpubReaderPercentSelectionActivity.h"
+
+#include <GfxRenderer.h>
+
+#include "MappedInputManager.h"
+#include "components/UITheme.h"
+#include "fontIds.h"
+
+namespace {
+// Fine/coarse slider step sizes for percent adjustments.
+constexpr int kSmallStep = 1;
+constexpr int kLargeStep = 10;
+}  // namespace
+
+void EpubReaderPercentSelectionActivity::onEnter() {
+  ActivityWithSubactivity::onEnter();
+  // Set up rendering task and mark first frame dirty.
+  renderingMutex = xSemaphoreCreateMutex();
+  updateRequired = true;
+  xTaskCreate(&EpubReaderPercentSelectionActivity::taskTrampoline, "EpubPercentSlider", 4096, this, 1,
+              &displayTaskHandle);
+}
+
+void EpubReaderPercentSelectionActivity::onExit() {
+  ActivityWithSubactivity::onExit();
+  // Ensure the render task is stopped before freeing the mutex.
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+}
+
+void EpubReaderPercentSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<EpubReaderPercentSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void EpubReaderPercentSelectionActivity::displayTaskLoop() {
+  while (true) {
+    // Render only when the view is dirty and no subactivity is running.
+    if (updateRequired && !subActivity) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      renderScreen();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void EpubReaderPercentSelectionActivity::adjustPercent(const int delta) {
+  // Apply delta and clamp within 0-100.
+  percent += delta;
+  if (percent < 0) {
+    percent = 0;
+  } else if (percent > 100) {
+    percent = 100;
+  }
+  updateRequired = true;
+}
+
+void EpubReaderPercentSelectionActivity::loop() {
+  if (subActivity) {
+    subActivity->loop();
+    return;
+  }
+
+  // Back cancels, confirm selects, arrows adjust the percent.
+  if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+    onCancel();
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+    onSelect(percent);
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Left)) {
+    adjustPercent(-kSmallStep);
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Right)) {
+    adjustPercent(kSmallStep);
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Up)) {
+    adjustPercent(kLargeStep);
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Down)) {
+    adjustPercent(-kLargeStep);
+    return;
+  }
+}
+
+void EpubReaderPercentSelectionActivity::renderScreen() {
+  renderer.clearScreen();
+
+  // Title and numeric percent value.
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, "Go to Position", true, EpdFontFamily::BOLD);
+
+  const std::string percentText = std::to_string(percent) + "%";
+  renderer.drawCenteredText(UI_12_FONT_ID, 90, percentText.c_str(), true, EpdFontFamily::BOLD);
+
+  // Draw slider track.
+  const int screenWidth = renderer.getScreenWidth();
+  constexpr int barWidth = 360;
+  constexpr int barHeight = 16;
+  const int barX = (screenWidth - barWidth) / 2;
+  const int barY = 140;
+
+  renderer.drawRect(barX, barY, barWidth, barHeight);
+
+  // Fill slider based on percent.
+  const int fillWidth = (barWidth - 4) * percent / 100;
+  if (fillWidth > 0) {
+    renderer.fillRect(barX + 2, barY + 2, fillWidth, barHeight - 4);
+  }
+
+  // Draw a simple knob centered at the current percent.
+  const int knobX = barX + 2 + fillWidth - 2;
+  renderer.fillRect(knobX, barY - 4, 4, barHeight + 8, true);
+
+  // Hint text for step sizes.
+  renderer.drawCenteredText(SMALL_FONT_ID, barY + 30, "Left/Right: 1%  Up/Down: 10%", true);
+
+  // Button hints follow the current front button layout.
+  const auto labels = mappedInput.mapLabels("« Back", "Select", "-", "+");
+  GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  renderer.displayBuffer();
+}