Make backend API URL configurable via environment variables

Updated frontend to use BACKEND_API_URL environment variable at compile time with fallback to localhost. Added configuration to Trunk.toml and .env.example for development.

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

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

Trunk.toml

@@ -2,6 +2,9 @@
 target = "index.html"
 dist = "dist"
 
+[env]
+BACKEND_API_URL = "http://localhost:3000/api"
+
 [watch]
 watch = ["src", "Cargo.toml"]
 ignore = ["backend/"]