feat(analytics): useFeatureFlag composable #3771

Context

Phase 4 PostHog — depends on PostHog fix issue.

What

• useFeatureFlag(flagKey) composable returning reactive ref (boolean or string variant)
• usePostHog() composable for direct access to posthog instance
• Handles: flags not loaded yet (default false), posthog not configured (always false)
• onFeatureFlags callback to update reactive state when flags arrive

Acceptance criteria

• useFeatureFlag returns reactive ref, updates when flags load
• Works with boolean and multivariate flags
• Returns false if posthog not configured
• usePostHog() returns posthog instance or null
• Unit tests