feat: Add CSS parsing and CSS support in EPUBs (#411)

## Summary

* **What is the goal of this PR?**

- Adds basic CSS parsing to EPUBs and determine the CSS rules when
rendering to the screen so that text is styled correctly. Currently
supports bold, underline, italics, margin, padding, and text alignment

## Additional Context

- My main reason for wanting this is that the book I'm currently
reading, Carl's Doomsday Scenario (2nd in the Dungeon Crawler Carl
series), relies _a lot_ on styled text for telling parts of the story.
When text is bolded, it's supposed to be a message that's rendered
"on-screen" in the story. When characters are "chatting" with each
other, the text is bolded and their names are underlined. Plus, normal
emphasis is provided with italicizing words here and there. So, this
greatly improves my experience reading this book on the Xteink, and I
figured it was useful enough for others too.
- For transparency: I'm a software engineer, but I'm mostly frontend and
TypeScript/JavaScript. It's been _years_ since I did any C/C++, so I
would not be surprised if I'm doing something dumb along the way in this
code. Please don't hesitate to ask for changes if something looks off. I
heavily relied on Claude Code for help, and I had a lot of inspiration
from how [microreader](https://github.com/CidVonHighwind/microreader)
achieves their CSS parsing and styling. I did give this as good of a
code review as I could and went through everything, and _it works on my
machine_ 😄

### Before

![IMG_6271](https://github.com/user-attachments/assets/dba7554d-efb6-4d13-88bc-8b83cd1fc615)

![IMG_6272](https://github.com/user-attachments/assets/61ba2de0-87c9-4f39-956f-013da4fe20a4)

### After

![IMG_6268](https://github.com/user-attachments/assets/ebe11796-cca9-4a46-b9c7-0709c7932818)

![IMG_6269](https://github.com/user-attachments/assets/e89c33dc-ff47-4bb7-855e-863fe44b3202)

---

### AI Usage

Did you use AI tools to help write this code? **YES**, Claude Code

lib/Epub/Epub.cpp

@@ -86,6 +86,10 @@ bool Epub::parseContentOpf(BookMetadataCache::BookMetadata& bookMetadata) {
     tocNavItem = opfParser.tocNavPath;
   }
 
+  if (!opfParser.cssFiles.empty()) {
+    cssFiles = opfParser.cssFiles;
+  }
+
   Serial.printf("[%lu] [EBP] Successfully parsed content.opf\n", millis());
   return true;
 }
@@ -204,15 +208,91 @@ bool Epub::parseTocNavFile() const {
   return true;
 }
 
+std::string Epub::getCssRulesCache() const { return cachePath + "/css_rules.cache"; }
+
+bool Epub::loadCssRulesFromCache() const {
+  FsFile cssCacheFile;
+  if (SdMan.openFileForRead("EBP", getCssRulesCache(), cssCacheFile)) {
+    if (cssParser->loadFromCache(cssCacheFile)) {
+      cssCacheFile.close();
+      Serial.printf("[%lu] [EBP] Loaded CSS rules from cache\n", millis());
+      return true;
+    }
+    cssCacheFile.close();
+    Serial.printf("[%lu] [EBP] CSS cache invalid, reparsing\n", millis());
+  }
+  return false;
+}
+
+void Epub::parseCssFiles() const {
+  if (cssFiles.empty()) {
+    Serial.printf("[%lu] [EBP] No CSS files to parse, but CssParser created for inline styles\n", millis());
+  }
+
+  // Try to load from CSS cache first
+  if (!loadCssRulesFromCache()) {
+    // Cache miss - parse CSS files
+    for (const auto& cssPath : cssFiles) {
+      Serial.printf("[%lu] [EBP] Parsing CSS file: %s\n", millis(), cssPath.c_str());
+
+      // Extract CSS file to temp location
+      const auto tmpCssPath = getCachePath() + "/.tmp.css";
+      FsFile tempCssFile;
+      if (!SdMan.openFileForWrite("EBP", tmpCssPath, tempCssFile)) {
+        Serial.printf("[%lu] [EBP] Could not create temp CSS file\n", millis());
+        continue;
+      }
+      if (!readItemContentsToStream(cssPath, tempCssFile, 1024)) {
+        Serial.printf("[%lu] [EBP] Could not read CSS file: %s\n", millis(), cssPath.c_str());
+        tempCssFile.close();
+        SdMan.remove(tmpCssPath.c_str());
+        continue;
+      }
+      tempCssFile.close();
+
+      // Parse the CSS file
+      if (!SdMan.openFileForRead("EBP", tmpCssPath, tempCssFile)) {
+        Serial.printf("[%lu] [EBP] Could not open temp CSS file for reading\n", millis());
+        SdMan.remove(tmpCssPath.c_str());
+        continue;
+      }
+      cssParser->loadFromStream(tempCssFile);
+      tempCssFile.close();
+      SdMan.remove(tmpCssPath.c_str());
+    }
+
+    // Save to cache for next time
+    FsFile cssCacheFile;
+    if (SdMan.openFileForWrite("EBP", getCssRulesCache(), cssCacheFile)) {
+      cssParser->saveToCache(cssCacheFile);
+      cssCacheFile.close();
+    }
+
+    Serial.printf("[%lu] [EBP] Loaded %zu CSS style rules from %zu files\n", millis(), cssParser->ruleCount(),
+                  cssFiles.size());
+  }
+}
+
 // load in the meta data for the epub file
-bool Epub::load(const bool buildIfMissing) {
+bool Epub::load(const bool buildIfMissing, const bool skipLoadingCss) {
   Serial.printf("[%lu] [EBP] Loading ePub: %s\n", millis(), filepath.c_str());
 
   // Initialize spine/TOC cache
   bookMetadataCache.reset(new BookMetadataCache(cachePath));
+  // Always create CssParser - needed for inline style parsing even without CSS files
+  cssParser.reset(new CssParser());
 
   // Try to load existing cache first
   if (bookMetadataCache->load()) {
+    if (!skipLoadingCss && !loadCssRulesFromCache()) {
+      Serial.printf("[%lu] [EBP] Warning: CSS rules cache not found, attempting to parse CSS files\n", millis());
+      // to get CSS file list
+      if (!parseContentOpf(bookMetadataCache->coreMetadata)) {
+        Serial.printf("[%lu] [EBP] Could not parse content.opf from cached bookMetadata for CSS files\n", millis());
+        // continue anyway - book will work without CSS and we'll still load any inline style CSS
+      }
+      parseCssFiles();
+    }
     Serial.printf("[%lu] [EBP] Loaded ePub: %s\n", millis(), filepath.c_str());
     return true;
   }
@@ -309,6 +389,11 @@ bool Epub::load(const bool buildIfMissing) {
     return false;
   }
 
+  if (!skipLoadingCss) {
+    // Parse CSS files after cache reload
+    parseCssFiles();
+  }
+
   Serial.printf("[%lu] [EBP] Loaded ePub: %s\n", millis(), filepath.c_str());
   return true;
 }