desktop: rustfmt the client-header tuple

Applied verbatim from run 2886's cargo fmt --check diff.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SreJkbxB4gx8pPsu8QbLPi

desktop/src-tauri/src/sync/compat.rs

@@ -159,7 +159,10 @@ pub fn client_headers() -> [(&'static str, String); 2] {
     let agent = format!("thoughtsync-desktop/{}", env!("CARGO_PKG_VERSION"));
     [
         ("X-ThoughtSync-Client", agent),
-        ("X-ThoughtSync-Protocol", CLIENT_PROTOCOL_VERSION.to_string()),
+        (
+            "X-ThoughtSync-Protocol",
+            CLIENT_PROTOCOL_VERSION.to_string(),
+        ),
     ]
 }