fix: Destroy CSS Cache file when invalid (#1018)

## Summary

* Destroy CSS Cache file when invalid

## Additional Context

* Fixes issue where it would attempt to rebuild every book open

---

### AI Usage

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Did you use AI tools to help write this code? No

lib/Epub/Epub.cpp

@@ -268,8 +268,14 @@ void Epub::parseCssFiles() const {
     LOG_DBG("EBP", "No CSS files to parse, but CssParser created for inline styles");
   }
 
+  LOG_DBG("EBP", "CSS files to parse: %zu", cssFiles.size());
+
   // See if we have a cached version of the CSS rules
-  if (!cssParser->hasCache()) {
+  if (cssParser->hasCache()) {
+    LOG_DBG("EBP", "CSS cache exists, skipping parseCssFiles");
+    return;
+  }
+
   // No cache yet - parse CSS files
   for (const auto& cssPath : cssFiles) {
     LOG_DBG("EBP", "Parsing CSS file: %s", cssPath.c_str());
@@ -325,7 +331,6 @@ void Epub::parseCssFiles() const {
   cssParser->clear();
 
   LOG_DBG("EBP", "Loaded %zu CSS style rules from %zu files", cssParser->ruleCount(), cssFiles.size());
-  }
 }
 
 // load in the meta data for the epub file
@@ -341,12 +346,10 @@ bool Epub::load(const bool buildIfMissing, const bool skipLoadingCss) {
   if (bookMetadataCache->load()) {
     if (!skipLoadingCss) {
       // Rebuild CSS cache when missing or when cache version changed (loadFromCache removes stale file)
-      bool needCssRebuild = !cssParser->hasCache();
-      if (cssParser->hasCache() && !cssParser->loadFromCache()) {
-        needCssRebuild = true;
-      }
-      if (needCssRebuild) {
+      if (!cssParser->hasCache() || !cssParser->loadFromCache()) {
         LOG_DBG("EBP", "CSS rules cache missing or stale, attempting to parse CSS files");
+        cssParser->deleteCache();
+
         if (!parseContentOpf(bookMetadataCache->coreMetadata)) {
           LOG_ERR("EBP", "Could not parse content.opf from cached bookMetadata for CSS files");
           // continue anyway - book will work without CSS and we'll still load any inline style CSS

lib/Epub/Epub/css/CssParser.cpp

@@ -640,6 +640,10 @@ constexpr char rulesCache[] = "/css_rules.cache";
 
 bool CssParser::hasCache() const { return Storage.exists((cachePath + rulesCache).c_str()); }
 
+void CssParser::deleteCache() const {
+  if (hasCache()) Storage.remove((cachePath + rulesCache).c_str());
+}
+
 bool CssParser::saveToCache() const {
   if (cachePath.empty()) {
     return false;

lib/Epub/Epub/css/CssParser.h

@@ -85,6 +85,11 @@ class CssParser {
    */
   bool hasCache() const;
 
+  /**
+   * Delete CSS rules cache file exists
+   */
+  void deleteCache() const;
+
   /**
    * Save parsed CSS rules to a cache file.
    * @return true if cache was written successfully