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>

.env.example

@@ -9,5 +9,9 @@ CALDAV_CALENDAR_PATH=/calendars/your-username/personal/
 # Optional: Task/Todo collection path
 CALDAV_TASKS_PATH=/calendars/your-username/tasks/
 
+# Backend API Configuration
+# Set this to point to your backend API server
+BACKEND_API_URL=http://localhost:3000/api
+
 # Development settings
 RUST_LOG=info

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/"]

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
+        // Get backend URL from environment variable at compile time, fallback to localhost
-        Self {
+        let base_url = option_env!("BACKEND_API_URL")
-            base_url: "http://localhost:3000/api".to_string(),
+            .unwrap_or("http://localhost:3000/api")
-        }
+            .to_string();
+        
+        Self { base_url }
     }
 
     pub async fn register(&self, request: RegisterRequest) -> Result<AuthResponse, String> {