fix: Fix img layout issue / support CSS display:none for elements and images (#1443)

## Summary
- Add CSS `display: none` support to the EPUB rendering pipeline (fixes
#1431)
- Parse `display` property in stylesheets and inline styles, with full
cascade resolution (element, class, element.class, inline)
- Skip hidden elements and all their descendants in
`ChapterHtmlSlimParser`
- Separate display:none check for `<img>` tags (image code path is
independent of the general element handler)
- Flush pending text blocks before placing images to fix layout ordering
(text preceding an image now correctly renders above it)
- Bump CSS cache version to 4 to invalidate stale caches
- Add test EPUB (`test_display_none.epub`) covering class selectors,
element selectors, combined selectors, inline styles, nested hidden
content, hidden images, style priority/override, and realistic use cases

lib/Epub/Epub/css/CssStyle.h

@@ -54,6 +54,9 @@ enum class CssFontWeight : uint8_t { Normal = 0, Bold = 1 };
 // Text decoration options
 enum class CssTextDecoration : uint8_t { None = 0, Underline = 1 };
 
+// Display options - only None and Block are relevant for e-ink rendering
+enum class CssDisplay : uint8_t { Block = 0, None = 1 };
+
 // Bitmask for tracking which properties have been explicitly set
 struct CssPropertyFlags {
   uint16_t textAlign : 1;
@@ -71,6 +74,7 @@ struct CssPropertyFlags {
   uint16_t paddingRight : 1;
   uint16_t imageHeight : 1;
   uint16_t imageWidth : 1;
+  uint16_t display : 1;
 
   CssPropertyFlags()
       : textAlign(0),
@@ -87,19 +91,20 @@ struct CssPropertyFlags {
         paddingLeft(0),
         paddingRight(0),
         imageHeight(0),
-        imageWidth(0) {}
+        imageWidth(0),
+        display(0) {}
 
   [[nodiscard]] bool anySet() const {
     return textAlign || fontStyle || fontWeight || textDecoration || textIndent || marginTop || marginBottom ||
            marginLeft || marginRight || paddingTop || paddingBottom || paddingLeft || paddingRight || imageHeight ||
-           imageWidth;
+           imageWidth || display;
   }
 
   void clearAll() {
     textAlign = fontStyle = fontWeight = textDecoration = textIndent = 0;
     marginTop = marginBottom = marginLeft = marginRight = 0;
     paddingTop = paddingBottom = paddingLeft = paddingRight = 0;
-    imageHeight = imageWidth = 0;
+    imageHeight = imageWidth = display = 0;
   }
 };
 
@@ -123,6 +128,7 @@ struct CssStyle {
   CssLength paddingRight;   // Padding right
   CssLength imageHeight;    // Height for img (e.g. 2em) – width derived from aspect ratio when only height set
   CssLength imageWidth;     // Width for img when both or only width set
+  CssDisplay display = CssDisplay::Block;  // display property (Block or None)
 
   CssPropertyFlags defined;  // Tracks which properties were explicitly set
 
@@ -189,6 +195,10 @@ struct CssStyle {
       imageWidth = base.imageWidth;
       defined.imageWidth = 1;
     }
+    if (base.hasDisplay()) {
+      display = base.display;
+      defined.display = 1;
+    }
   }
 
   [[nodiscard]] bool hasTextAlign() const { return defined.textAlign; }
@@ -206,6 +216,7 @@ struct CssStyle {
   [[nodiscard]] bool hasPaddingRight() const { return defined.paddingRight; }
   [[nodiscard]] bool hasImageHeight() const { return defined.imageHeight; }
   [[nodiscard]] bool hasImageWidth() const { return defined.imageWidth; }
+  [[nodiscard]] bool hasDisplay() const { return defined.display; }
 
   void reset() {
     textAlign = CssTextAlign::Left;
@@ -216,6 +227,7 @@ struct CssStyle {
     marginTop = marginBottom = marginLeft = marginRight = CssLength{};
     paddingTop = paddingBottom = paddingLeft = paddingRight = CssLength{};
     imageHeight = imageWidth = CssLength{};
+    display = CssDisplay::Block;
     defined.clearAll();
   }
 };