feat(app): support running embedded in a host (embeddedPreview + injection)

[SERDUN]

Overview

Let webtrit_phone run embedded inside another Flutter host — the theme configurator's realtime preview mounts the real app in-process. Two parts, both in bootstrap.dart / main.dart (RootApp) / app/view/app.dart (App):

1. Firebase-free bootstrap. bootstrap({bool embeddedPreview = false}): when embedded, skip Firebase app/messaging/local-push init, use SharedPreferencesAppIdProvider instead of the Installations-backed FirebaseAppIdProvider, and skip Firebase Remote Config. The host owns the default Firebase app (which may be misconfigured or absent — locally the configurator's app has an empty projectId), so any Firebase touch in the embedded app would throw.
2. External config injection. RootApp/App accept external ThemeSettings / ThemeMode / FeatureAccess streams (+ initial seeds) so the host drives the live appearance and feature/login/tab config. Theme is applied via AppThemeSettingsChanged / AppThemeModeChanged (no prefs write); the FeatureAccess StreamProvider source is swapped to the external stream. The analytics provider is made lazy and the analytics navigator observer is skipped when embedded, so FirebaseAnalytics.instance is never touched.

Why

Part of the configurator realtime-preview epic (depends on the EnvironmentConfig runtime-override PR for the dart-define injection path the host uses). Standalone builds are unaffected.

Notes

• Standalone behaviour byte-identical: embeddedPreview defaults to false, all external streams default to null.
• flutter analyze lib green.

[SERDUN]

Splitting into two PRs: (1) embeddedPreview Firebase-free bootstrap, (2) external Theme/FeatureAccess injection.