Add some varied default song covers

frontend/build.rs

@@ -0,0 +1,26 @@
+use std::fs;
+use std::path::Path;
+use std::io;
+
+fn main() {
+    index_default_covers().expect("index default covers");
+}
+
+/// Index all pngs in static/images/default_covers and expose the list in the build as an
+/// environment variable DEFAULT_SONG_COVERS. This list includes the path to the images so the
+/// frontend can fetch them.
+fn index_default_covers() -> io::Result<()> {
+    let mut files = vec![];
+    for dir in fs::read_dir("static/images/default_covers")? {
+        let path = dir?.path();
+        if path.extension().and_then(|s| s.to_str()) == Some("png") {
+            let path = Path::new("/").join(path.strip_prefix("static").unwrap());
+            files.push(path.to_string_lossy().to_string());
+        }
+    }
+
+    let list = files.join(",");
+    println!("cargo:rustc-env=DEFAULT_SONG_COVERS={list}");
+
+    Ok(())
+}