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/parsers/ChapterHtmlSlimParser.h

@@ -8,6 +8,8 @@
 
 #include "../ParsedText.h"
 #include "../blocks/TextBlock.h"
+#include "../css/CssParser.h"
+#include "../css/CssStyle.h"
 
 class Page;
 class GfxRenderer;
@@ -23,6 +25,7 @@ class ChapterHtmlSlimParser {
   int skipUntilDepth = INT_MAX;
   int boldUntilDepth = INT_MAX;
   int italicUntilDepth = INT_MAX;
+  int underlineUntilDepth = INT_MAX;
   // buffer for building up words from characters, will auto break if longer than this
   // leave one char at end for null pointer
   char partWordBuffer[MAX_WORD_SIZE + 1] = {};
@@ -37,8 +40,23 @@ class ChapterHtmlSlimParser {
   uint16_t viewportWidth;
   uint16_t viewportHeight;
   bool hyphenationEnabled;
+  const CssParser* cssParser;
 
-  void startNewTextBlock(TextBlock::Style style);
+  // Style tracking (replaces depth-based approach)
+  struct StyleStackEntry {
+    int depth = 0;
+    bool hasBold = false, bold = false;
+    bool hasItalic = false, italic = false;
+    bool hasUnderline = false, underline = false;
+  };
+  std::vector<StyleStackEntry> inlineStyleStack;
+  CssStyle currentCssStyle;
+  bool effectiveBold = false;
+  bool effectiveItalic = false;
+  bool effectiveUnderline = false;
+
+  void updateEffectiveInlineStyle();
+  void startNewTextBlock(const BlockStyle& blockStyle);
   void flushPartWordBuffer();
   void makePages();
   // XML callbacks
@@ -52,7 +70,8 @@ class ChapterHtmlSlimParser {
                                  const uint8_t paragraphAlignment, const uint16_t viewportWidth,
                                  const uint16_t viewportHeight, const bool hyphenationEnabled,
                                  const std::function<void(std::unique_ptr<Page>)>& completePageFn,
-                                 const std::function<void()>& popupFn = nullptr)
+                                 const std::function<void()>& popupFn = nullptr, const CssParser* cssParser = nullptr)
+
       : filepath(filepath),
         renderer(renderer),
         fontId(fontId),
@@ -63,7 +82,9 @@ class ChapterHtmlSlimParser {
         viewportHeight(viewportHeight),
         hyphenationEnabled(hyphenationEnabled),
         completePageFn(completePageFn),
-        popupFn(popupFn) {}
+        popupFn(popupFn),
+        cssParser(cssParser) {}
+
   ~ChapterHtmlSlimParser() = default;
   bool parseAndBuildPages();
   void addLineToPage(std::shared_ptr<TextBlock> line);