fix: Re-apply upstream PRs #1005, #1010, #1003

Re-applies changes that were accidentally discarded during a prior
dry-run cherry-pick reset (git checkout -- .).

- PR #1005: Use HalPowerManager for battery percentage (uint16_t return
  type, remove Battery.h, update theme files)
- PR #1010: Fix dangling pointer in onGoToReader()
- PR #1003: Render image placeholders while waiting for decode (adds
  isCached, renderPlaceholder, renderTextOnly, countUncachedImages,
  renderImagePlaceholders)

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/hal/HalPowerManager.cpp

@@ -78,7 +78,7 @@ void HalPowerManager::startDeepSleep(HalGPIO& gpio) const {
   esp_deep_sleep_start();
 }
 
-int HalPowerManager::getBatteryPercentage() const {
+uint16_t HalPowerManager::getBatteryPercentage() const {
   static const BatteryMonitor battery = BatteryMonitor(BAT_GPIO0);
   return battery.readPercentage();
 }