feat(app): let the caller supply theme and feature config sources

[SERDUN]

Overview

Makes RootApp agnostic about where its theme and feature config come from. It takes a ConfigSource (an {initial, updates} record) for each and wires it into the inherited StreamProvider — without reaching into the registry or branching on a host flag. The composition root decides the source.

This is the phone-side half of the embedding work (the configurator side comes next). It builds on the inherited-config groundwork already on develop, so the host only supplies a different source; consumers (App.build, ThemeModeScreen) stay agnostic.

Changes

typedef ConfigSource<T> = ({T initial, Stream<T> updates});

const RootApp({required this.instanceRegistry, required this.featureAccess, required this.themeSettings});
final ConfigSource<FeatureAccess> featureAccess;
final ConfigSource<ThemeSettings> themeSettings;

• RootApp.build wires each ConfigSource straight into a StreamProvider (initialData: source.initial, create: source.updates) — no registry access, no host ?? default branch.
• Standalone (main) resolves the sources from the bootstrap registry: the FeatureAccessStreamFactory stream, and the first AppThemes theme (static, so updates: const Stream.empty()).
• A host (the configurator's realtime preview) passes its own streams + bootstrap(firebase: const FirebaseIntegrationDisabled()) — live edits flow through the same inherited values.

It's a record, not a wrapper class; no imperative relay.

Verification

• flutter analyze — clean.
• Standalone behaviour is unchanged (registry-resolved sources; theme updates is empty).