v0.4.1: harden ISO uploads and beta UI polish

Add browser-safe chunked ISO uploads with progress, partial-file visibility, offset validation, and abort cleanup while keeping the legacy multipart endpoint for API clients.

Record host-log validation coverage, keep the queue/status UI copy clean, move release docs to 0.4.1, and tighten the dark theme to a near-black Netbox-style palette.

crates/iso-store/src/store.rs

@@ -216,6 +216,9 @@ impl IsoStore {
             return Err(Error::Invalid(format!("iso '{id}' already exists")));
         }
         let partial_path = self.iso_dir.join(format!("{id}.partial"));
+        if partial_path.exists() {
+            return Err(Error::Invalid(format!("iso '{id}' is already uploading")));
+        }
         let file = tokio::fs::File::create(&partial_path).await?;
         Ok(UploadHandle {
             id,
@@ -594,6 +597,19 @@ mod tests {
         assert!(matches!(r, Err(Error::Invalid(_))));
     }
 
+    #[tokio::test]
+    async fn begin_upload_rejects_existing_partial_file() {
+        let dir = tempdir().unwrap();
+        let store = IsoStore::new(dir.path().to_path_buf());
+        store.ensure_dirs().await.unwrap();
+        tokio::fs::write(dir.path().join("ubuntu.partial"), b"in-flight")
+            .await
+            .unwrap();
+
+        let r = store.begin_upload("ubuntu.iso").await;
+        assert!(matches!(r, Err(Error::Invalid(_))));
+    }
+
     #[tokio::test]
     async fn password_persists_via_meta_json_for_local_isos() {
         // Hash makes it onto disk so it survives a restart.