fix: TOC navigation and pagination improvements

- Fix recalculate_pages missing heading_top_spacing in page height calculation
- Improve build_toc path matching: extract filename first, fall back to substring
- Filter out EPUB3 nav.xhtml from content sections
- Skip Windows resource compilation when windres is not available
- Add unit tests for TOC filename matching and nav filtering

build.rs

@@ -1,6 +1,16 @@
 #[cfg(target_os = "windows")]
 fn main() {
-    embed_resource::compile("read.rc", embed_resource::NONE);
+    let windres_available = std::process::Command::new("where")
+        .arg("windres")
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false);
+
+    if windres_available {
+        embed_resource::compile("read.rc", embed_resource::NONE);
+    } else {
+        println!("cargo:warning=windres not found, skipping Windows resource (icon) compilation");
+    }
 }
 
 #[cfg(not(target_os = "windows"))]