fix: improve error message when sidecar and media arguments are swapped

src/bin/signedshot.rs

@@ -84,6 +84,27 @@ fn validate_command(sidecar_path: &Path, media_path: &Path, json_output: bool) -
             s
         }
         Err(e) => {
+            let err_msg = format!("{}", e);
+            if err_msg.contains("valid UTF-8") || err_msg.contains("expected value") {
+                let ext = sidecar_path
+                    .extension()
+                    .and_then(|e| e.to_str())
+                    .unwrap_or("");
+                if matches!(
+                    ext,
+                    "jpg" | "jpeg" | "png" | "heic" | "heif" | "mp4" | "mov"
+                ) {
+                    eprintln!(
+                        "[FAILED] The sidecar path appears to be a media file, not a JSON sidecar."
+                    );
+                    eprintln!("         Did you swap the arguments?");
+                    eprintln!("         Usage: signedshot validate <sidecar.json> <media>");
+                    return Err(anyhow::anyhow!(
+                        "Arguments appear to be swapped: sidecar path '{}' looks like a media file",
+                        sidecar_path.display()
+                    ));
+                }
+            }
             println!("[FAILED] Sidecar parsing: {}", e);
             return Err(e).context("Failed to parse sidecar");
         }