Add PostHog analytics to replace removed Firebase

Wire PostHog iOS SDK into existing EventLogger pattern so all 60+
call sites flow to self-hosted PostHog instance with zero changes.
Sets subscription person properties for segmentation on foreground.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Shared/EventLogger.swift

@@ -6,14 +6,17 @@
 //
 
 import Foundation
-// import Firebase  // Firebase removed
+#if canImport(PostHog)
+import PostHog
+#endif
 
 class EventLogger {
     static func log(event: String, withData data: [String: Any]? = nil) {
-        // Firebase Analytics disabled
-        // Analytics.logEvent(event, parameters: data)
         #if DEBUG
         print("[EventLogger] \(event)", data ?? "")
         #endif
+        #if canImport(PostHog)
+        PostHogSDK.shared.capture(event, properties: data)
+        #endif
     }
 }