feat: implement PostHog authentication hook

- Create usePostHogAuth hook for automatic user identification
- Add session-based identification tracking to prevent duplicate calls
- Implement proper cleanup of PocketBase auth listeners
- Follow PostHog best practices for identify timing
- Integrate with centralized PostHog utility functions

web/src/hooks/use-posthog-auth.ts

@@ -0,0 +1,42 @@
+"use client"
+
+import { usePostHog } from "posthog-js/react"
+import { useEffect, useRef } from "react"
+import { pb } from "@/lib/pb"
+import { identifyUserInPostHog, resetPostHogIdentity } from "@/lib/posthog-utils"
+
+export function usePostHogAuth() {
+	const posthog = usePostHog()
+	const hasIdentified = useRef(false)
+
+	useEffect(() => {
+		const checkAuthAndIdentify = () => {
+			if (pb.authStore.isValid && pb.authStore.model) {
+				// User is logged in, identify them in PostHog
+				// Only call identify once per session to avoid unnecessary calls
+				if (!hasIdentified.current) {
+					identifyUserInPostHog(posthog)
+					hasIdentified.current = true
+				}
+			} else {
+				// User is not logged in, reset PostHog identity
+				// This unlinks future events from the user (important for shared computers)
+				resetPostHogIdentity(posthog)
+				hasIdentified.current = false
+			}
+		}
+
+		// Check auth state on mount
+		checkAuthAndIdentify()
+
+		// Listen for auth changes
+		const unsubscribe = pb.authStore.onChange(() => {
+			checkAuthAndIdentify()
+		})
+
+		// Cleanup listener on unmount
+		return () => {
+			unsubscribe()
+		}
+	}, [posthog])
+}