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.

pubspec.yaml

@@ -1,7 +1,7 @@
 name: m8chat_app
 description: "M8Chat — Matrix chat client for Android, iOS, and Web."
 publish_to: 'none'
-version: 1.11.0+15
+version: 1.12.0+16
 
 environment:
   sdk: '>=3.11.0 <4.0.0'