feat(analytics): port SessionJourney tracking from GardenBuddy (v1.12.0+16)

Rolls the session/journey/friction tracking module out to M8Chat app2, per
plan ancient-petting-hejlsberg.md §5 step 4. Same pattern as GardenBuddy's
same-day port: SessionJourney.init() wired after WidgetsFlutterBinding.
ensureInitialized() and after the existing FlutterError.onError /
PlatformDispatcher.instance.onError handlers, which it chains rather than
replaces. SessionJourneyNavigatorObserver registered on the GoRouter
instance; SessionJourneyGestureLayer wraps MaterialApp.router for rage-tap
detection.

This repo has no shared offline-cache module like GardenBuddy's
core/offline/, so persistence is a small scoped sj_local_store.dart using
the package:web dependency already present in the repo (localStorage on
web, in-memory fallback for the test VM), rather than adding a new package.

app_key m8chat.au-app was reserved today in track.php v2.0.1's
allowed_sites/allowed_origins for https://app2.m8chat.au, confirmed by grep
before use.

Verified live: build deployed to app2.m8chat.au (md5-matched), Playwright
navigation produced real session/visitor/screen_view rows in
admin_flutterstats with app_key=m8chat.au-app, nginx logs show no 5xx and
no real visitor served a broken build during the test window.

lib/app/router.dart

@@ -1,12 +1,15 @@
-// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-05
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-09-25
 // All route definitions in one place.
 // GoRouter is created ONCE (keepAlive) and re-evaluates redirects via
 // refreshListenable — avoids the GoRouter recreation bug from ref.watch.
+// v1.2.0: registered SessionJourneyNavigatorObserver (session/journey
+// tracking, plan ancient-petting-hejlsberg.md §3/§5).
 
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
+import '../core/analytics/session_journey.dart';
 import '../core/auth/auth_notifier.dart';
 import '../core/auth/auth_state.dart';
 import '../features/auth/presentation/login_screen.dart';
@@ -40,11 +43,7 @@ class _AuthRefreshNotifier extends ChangeNotifier {
 }
 
 /// Routes that are accessible without authentication.
-const _publicRoutes = {
-  AppRoutes.welcome,
-  AppRoutes.login,
-  AppRoutes.jitsi,
-};
+const _publicRoutes = {AppRoutes.welcome, AppRoutes.login, AppRoutes.jitsi};
 
 @Riverpod(keepAlive: true)
 GoRouter router(Ref ref) {
@@ -57,6 +56,7 @@ GoRouter router(Ref ref) {
     initialLocation: AppRoutes.root,
     refreshListenable: notifier,
     debugLogDiagnostics: false,
+    observers: [SessionJourneyNavigatorObserver()],
     redirect: (BuildContext context, GoRouterState state) {
       // Read (not watch) so redirect does not itself trigger provider changes.
       final authState = ref.read(authProvider);