Update .env.example, tsconfig.json, and PHASE-TRACKING.md for WebUI integration and feature completion

- Added WEBUI_URL and SESSION_SECRET to .env.example for improved configuration.
- Updated TypeScript target in apps/webui/tsconfig.json to ES2022 for modern features.
- Revised PHASE-TRACKING.md to reflect the completion of Phase 6, detailing implemented features and pending manual tests for the WebUI, including Discord OAuth2 integration and session management.

apps/webui/tsconfig.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "target": "ES2017",
+    "target": "ES2022",
     "lib": ["dom", "dom.iterable", "esnext"],
     "allowJs": true,
     "skipLibCheck": true,