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/css/CssParser.h

@@ -0,0 +1,114 @@
+#pragma once
+
+#include <SdFat.h>
+
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "CssStyle.h"
+
+/**
+ * Lightweight CSS parser for EPUB stylesheets
+ *
+ * Parses CSS files and extracts styling information relevant for e-ink display.
+ * Uses a two-phase approach: first tokenizes the CSS content, then builds
+ * a rule database that can be queried during HTML parsing.
+ *
+ * Supported selectors:
+ *   - Element selectors: p, div, h1, etc.
+ *   - Class selectors: .classname
+ *   - Combined: element.classname
+ *   - Grouped: selector1, selector2 { }
+ *
+ * Not supported (silently ignored):
+ *   - Descendant/child selectors
+ *   - Pseudo-classes and pseudo-elements
+ *   - Media queries (content is skipped)
+ *   - @import, @font-face, etc.
+ */
+class CssParser {
+ public:
+  CssParser() = default;
+  ~CssParser() = default;
+
+  // Non-copyable
+  CssParser(const CssParser&) = delete;
+  CssParser& operator=(const CssParser&) = delete;
+
+  /**
+   * Load and parse CSS from a file stream.
+   * Can be called multiple times to accumulate rules from multiple stylesheets.
+   * @param source Open file handle to read from
+   * @return true if parsing completed (even if no rules found)
+   */
+  bool loadFromStream(FsFile& source);
+
+  /**
+   * Look up the style for an HTML element, considering tag name and class attributes.
+   * Applies CSS cascade: element style < class style < element.class style
+   *
+   * @param tagName The HTML element name (e.g., "p", "div")
+   * @param classAttr The class attribute value (may contain multiple space-separated classes)
+   * @return Combined style with all applicable rules merged
+   */
+  [[nodiscard]] CssStyle resolveStyle(const std::string& tagName, const std::string& classAttr) const;
+
+  /**
+   * Parse an inline style attribute string.
+   * @param styleValue The value of a style="" attribute
+   * @return Parsed style properties
+   */
+  [[nodiscard]] static CssStyle parseInlineStyle(const std::string& styleValue);
+
+  /**
+   * Check if any rules have been loaded
+   */
+  [[nodiscard]] bool empty() const { return rulesBySelector_.empty(); }
+
+  /**
+   * Get count of loaded rule sets
+   */
+  [[nodiscard]] size_t ruleCount() const { return rulesBySelector_.size(); }
+
+  /**
+   * Clear all loaded rules
+   */
+  void clear() { rulesBySelector_.clear(); }
+
+  /**
+   * Save parsed CSS rules to a cache file.
+   * @param file Open file handle to write to
+   * @return true if cache was written successfully
+   */
+  bool saveToCache(FsFile& file) const;
+
+  /**
+   * Load CSS rules from a cache file.
+   * Clears any existing rules before loading.
+   * @param file Open file handle to read from
+   * @return true if cache was loaded successfully
+   */
+  bool loadFromCache(FsFile& file);
+
+ private:
+  // Storage: maps normalized selector -> style properties
+  std::unordered_map<std::string, CssStyle> rulesBySelector_;
+
+  // Internal parsing helpers
+  void processRuleBlock(const std::string& selectorGroup, const std::string& declarations);
+  static CssStyle parseDeclarations(const std::string& declBlock);
+
+  // Individual property value parsers
+  static CssTextAlign interpretAlignment(const std::string& val);
+  static CssFontStyle interpretFontStyle(const std::string& val);
+  static CssFontWeight interpretFontWeight(const std::string& val);
+  static CssTextDecoration interpretDecoration(const std::string& val);
+  static CssLength interpretLength(const std::string& val);
+  static int8_t interpretSpacing(const std::string& val);
+
+  // String utilities
+  static std::string normalized(const std::string& s);
+  static std::vector<std::string> splitOnChar(const std::string& s, char delimiter);
+  static std::vector<std::string> splitWhitespace(const std::string& s);
+};