feat: add PostHog product analytics for key user actions (#110)

Tracks 9 key events: account_created, login_success, flow_viewed,
session_started, session_completed, export_generated, ai_feature_used,
psa_connected, session_shared. Identifies users on login, resets on
logout. Autocapture disabled — only explicit discrete events.

Set VITE_POSTHOG_KEY in environment to enable.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -37,6 +37,7 @@
     "immer": "^11.1.3",
     "lucide-react": "^0.563.0",
     "monaco-editor": "^0.55.1",
+    "posthog-js": "^1.360.2",
     "react": "^19.2.0",
     "react-day-picker": "^9.13.1",
     "react-dom": "^19.2.0",
@@ -50,8 +51,8 @@
     "zustand": "^5.0.10"
   },
   "devDependencies": {
-    "@playwright/test": "^1.55.0",
     "@eslint/js": "^9.39.1",
+    "@playwright/test": "^1.55.0",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
     "@testing-library/user-event": "^14.6.1",