chore: add .gitignore and remove tracked artifacts

Remove node_modules/, .DS_Store, .vscode/ and build output from
git tracking. Adds .gitignore to prevent future accidental commits
of dependencies and generated files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,20 @@
+# Dependencies
+node_modules/
+
+# Build output
+dist/
+backend/public/
+
+# Vite
+.vite/
+*.timestamp-*
+
+# OS
+.DS_Store
+
+# IDE
+.vscode/
+
+# Environment
+.env
+.env.*