chore: clarify gitignore pattern for server binary

Change 'server' to '/server' to make pattern more explicit.
This prevents editors from confusing the ignored server binary
with the tracked cmd/server/ source code directory.

Pattern now only matches:
- /server (binary at root, ignored)
- NOT cmd/server/ (source directory, tracked)

.gitignore

@@ -1,8 +1,7 @@
 # Build artifacts
 bookhoard
 bookhoard.*
-server
+/server
-server
 
 # Node.js
 node_modules/