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>

src/auth.rs

@@ -50,10 +50,12 @@ pub struct AuthService {
 
 impl AuthService {
     pub fn new() -> Self {
-        // Default to localhost backend - could be configurable via env var in the future
-        Self {
-            base_url: "http://localhost:3000/api".to_string(),
-        }
+        // Get backend URL from environment variable at compile time, fallback to localhost
+        let base_url = option_env!("BACKEND_API_URL")
+            .unwrap_or("http://localhost:3000/api")
+            .to_string();
+        
+        Self { base_url }
     }
 
     pub async fn register(&self, request: RegisterRequest) -> Result<AuthResponse, String> {