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>

src/main.cpp

@@ -15,7 +15,6 @@
 #include <cstring>
 #include <ctime>
 
-#include "Battery.h"
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "KOReaderCredentialStore.h"
@@ -227,9 +226,9 @@ void onGoHome();
 void onGoToMyLibraryWithPath(const std::string& path);
 void onGoToRecentBooks();
 void onGoToReader(const std::string& initialEpubPath) {
+  const std::string bookPath = initialEpubPath;  // Copy before exitActivity() invalidates the reference
   exitActivity();
-  enterNewActivity(
-      new ReaderActivity(renderer, mappedInputManager, initialEpubPath, onGoHome, onGoToMyLibraryWithPath));
+  enterNewActivity(new ReaderActivity(renderer, mappedInputManager, bookPath, onGoHome, onGoToMyLibraryWithPath));
 }
 
 void onGoToFileTransfer() {