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.h

@@ -28,7 +28,7 @@ class HalPowerManager {
   void startDeepSleep(HalGPIO& gpio) const;
 
   // Get battery percentage (range 0-100)
-  int getBatteryPercentage() const;
+  uint16_t getBatteryPercentage() const;
 
   // RAII lock to prevent low-power mode during critical work (e.g. rendering)
   class Lock {