Fix Nix build by removing accidentally committed build artifacts

- Remove server/_build/ and server/deps/ from git tracking
- Update .gitignore to properly ignore server build directories
- These directories caused symlink conflicts in Nix build

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.gitignore

@@ -1,11 +1,11 @@
 # The directory Mix will write compiled artifacts to.
-/_build/
+server/_build/
 
 # If you run "mix test --cover", coverage assets end up here.
-/cover/
+server/cover/
 
 # The directory Mix downloads your dependencies sources to.
-/deps/
+server/deps/
 
 # Where third-party dependencies like ExDoc output generated docs.
 /doc/