Fix: append .bak extension instead of replacing file extension

with_extension("md.bak") replaces the existing extension rather
than appending. For .md files this produces the correct result by
coincidence, but for .json files it would produce settings.md.bak
instead of settings.json.bak.

Use with_file_name(format!("{}.bak", file_name)) to properly
append .bak, matching the approach used in utils/backup.rs.

Relates to #195

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: prefrontalsys

src-tauri/src/services/gist_sync.rs

@@ -558,7 +558,8 @@ pub fn apply_pulled_payload(
             }
             // Back up existing file before overwriting
             if path.exists() {
-                let backup = path.with_extension("md.bak");
+                let file_name = path.file_name().unwrap_or_default().to_string_lossy();
+                let backup = path.with_file_name(format!("{}.bak", file_name));
                 let _ = std::fs::copy(&path, &backup);
             }
             std::fs::write(&path, content)?;