fix(webdav): upload the sync manifest as application/octet-stream

CSTCloud Data Capsule's WebDAV server answers PUT manifest.json with
500 "Uploaded object size does not match the upload session" whenever
the body is multi-line JSON sent as application/json. The same pretty
JSON is stored byte-for-byte when sent as application/octet-stream, and
db.sql / skills.zip already upload fine with their own types.

Send the manifest with application/octet-stream. The manifest bytes and
hash stay the same and readers never look at Content-Type, so old and
new versions keep reading each other's manifests. S3 sync is unchanged.

Refs #8554

src-tauri/src/services/webdav_sync.rs

@@ -31,6 +31,12 @@ pub(crate) fn sync_mutex() -> &'static tokio::sync::Mutex<()> {
 
 pub(crate) mod archive;
 
+/// Content type for the manifest PUT. Some WebDAV servers (e.g. CSTCloud
+/// Data Capsule) reject multi-line bodies sent as `application/json` with a
+/// size-mismatch 500, while accepting the same bytes as a generic binary.
+/// The bytes are unchanged, so readers of any version are unaffected.
+const MANIFEST_CONTENT_TYPE: &str = "application/octet-stream";
+
 struct RemoteSnapshot {
     layout: RemoteLayout,
     manifest: SyncManifest,
@@ -73,7 +79,7 @@ pub async fn upload(
         &manifest_url,
         &auth,
         snapshot.manifest_bytes,
-        "application/json",
+        MANIFEST_CONTENT_TYPE,
     )
     .await?;