feat: Add percentage support to CSS properties (#738)

## Summary
- Closes #730

**What is the goal of this PR?**
- Adds percentage-based value support to CSS properties that accept
percentages (padding, margin, text-indent)
 
**What changes are included?**
- Adds `Percent` as another CSS unit
- Passes the viewport width to `fromCssStyle` so that we can resolve
percentage-based values
- Adds a fallback of using an emspace for text-indent if we have an
unresolvable value for whatever reason

## Additional Context

- This was missed in my CSS support feature, and the fallback when we
encounter a percentage value is to use px instead. This means 5% (which
would be ~30px on the screen) turns into 5px. When percentages are used
in `text-indent`, this fallback behavior makes the indent look like a
single space character. Whoops! 😬

My test EPUB has been updated
[here](https://github.com/jdk2pq/css-test-epub) with percentage based
CSS values at the end of the book.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

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

lib/Epub/Epub/css/CssStyle.h

@@ -4,7 +4,7 @@
 
 // Matches order of PARAGRAPH_ALIGNMENT in CrossPointSettings
 enum class CssTextAlign : uint8_t { Justify = 0, Left = 1, Center = 2, Right = 3, None = 4 };
-enum class CssUnit : uint8_t { Pixels = 0, Em = 1, Rem = 2, Points = 3 };
+enum class CssUnit : uint8_t { Pixels = 0, Em = 1, Rem = 2, Points = 3, Percent = 4 };
 
 // Represents a CSS length value with its unit, allowing deferred resolution to pixels
 struct CssLength {
@@ -17,21 +17,32 @@ struct CssLength {
   // Convenience constructor for pixel values (most common case)
   explicit CssLength(const float pixels) : value(pixels) {}
 
+  // Returns true if this length can be resolved to pixels with the given context.
+  // Percentage units require a non-zero containerWidth to resolve.
+  [[nodiscard]] bool isResolvable(const float containerWidth = 0) const {
+    return unit != CssUnit::Percent || containerWidth > 0;
+  }
+
   // Resolve to pixels given the current em size (font line height)
-  [[nodiscard]] float toPixels(const float emSize) const {
+  // containerWidth is needed for percentage units (e.g. viewport width)
+  [[nodiscard]] float toPixels(const float emSize, const float containerWidth = 0) const {
     switch (unit) {
       case CssUnit::Em:
       case CssUnit::Rem:
         return value * emSize;
       case CssUnit::Points:
         return value * 1.33f;  // Approximate pt to px conversion
+      case CssUnit::Percent:
+        return value * containerWidth / 100.0f;
       default:
         return value;
     }
   }
 
   // Resolve to int16_t pixels (for BlockStyle fields)
-  [[nodiscard]] int16_t toPixelsInt16(const float emSize) const { return static_cast<int16_t>(toPixels(emSize)); }
+  [[nodiscard]] int16_t toPixelsInt16(const float emSize, const float containerWidth = 0) const {
+    return static_cast<int16_t>(toPixels(emSize, containerWidth));
+  }
 };
 
 // Font style options matching CSS font-style property