fix(#338): Standardize API base URL and auth mechanism across components

- Create centralized config module (apps/web/src/lib/config.ts) exporting:
  - API_BASE_URL: Main API server URL from NEXT_PUBLIC_API_URL
  - ORCHESTRATOR_URL: Orchestrator service URL from NEXT_PUBLIC_ORCHESTRATOR_URL
  - Helper functions for building full URLs
- Update client.ts to import from central config
- Update LoginButton.tsx to use API_BASE_URL from config
- Update useWebSocket.ts to use API_BASE_URL from config
- Update AgentStatusWidget.tsx to use ORCHESTRATOR_URL from config
- Update TaskProgressWidget.tsx to use ORCHESTRATOR_URL from config
- Update useGraphData.ts to use API_BASE_URL from config
  - Fixed wrong default port (was 8000, now uses correct 3001)
- Add comprehensive tests for config module
- Update useWebSocket tests to properly mock config module

Refs #338

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/components/auth/LoginButton.tsx

@@ -1,14 +1,13 @@
 "use client";
 
 import { Button } from "@mosaic/ui";
-
-const API_URL = process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:3001";
+import { API_BASE_URL } from "@/lib/config";
 
 export function LoginButton(): React.JSX.Element {
   const handleLogin = (): void => {
     // Redirect to the backend OIDC authentication endpoint
     // BetterAuth will handle the OIDC flow and redirect back to the callback
-    window.location.assign(`${API_URL}/auth/signin/authentik`);
+    window.location.assign(`${API_BASE_URL}/auth/signin/authentik`);
   };
 
   return (