Fix Material localizations after material_ui migration

[MangelSpec]

This was split out of #1153 following review so the background-fetch and localization fixes can be reviewed independently.

What was happening

After the material_ui migration, opening KitchenOwl with a non-English locale could show a red exception screen.

AppLocalizations.localizationsDelegates still contains the legacy delegates from flutter_localizations. Those do not provide the localization types expected by widgets from the decoupled material_ui package.

Emulator screenshot (German locale)

Exception and stack trace

No MaterialLocalizations found.
AppBar widgets require MaterialLocalizations to be provided by a
Localizations widget ancestor.

#0  debugCheckHasMaterialLocalizations
    (package:material_ui/src/debug.dart:97:7)
#1  debugCheckHasMaterialLocalizations
    (package:material_ui/src/debug.dart:117:4)
#2  _AppBarState.build
    (package:material_ui/src/app_bar.dart:946:12)
#3  StatefulElement.build
    (package:flutter/src/widgets/framework.dart:5944:27)
#4  ComponentElement.performRebuild
    (package:flutter/src/widgets/framework.dart:5830:15)
#5  StatefulElement.performRebuild
    (package:flutter/src/widgets/framework.dart:5995:11)

The runner also reported:

A MaterialLocalizations delegate that supports the de locale was not found.
A CupertinoLocalizations delegate that supports the de locale was not found.

What changed

• Keep AppLocalizations.delegate for KitchenOwl's translated messages.
• Register GlobalMaterialLocalizations.delegates from material_ui.
• Keep the locale-name delegate.
• Add a focused German localization regression test.

GlobalMaterialLocalizations.delegates includes the Material, Cupertino, and Widgets delegates required by the decoupled UI packages.

Verification

• Reproduced the red exception screen on an emulator using de-DE.
• Installed the fixed APK on the same emulator and confirmed the German login screen renders normally.
• Confirmed the fresh Flutter log contains no localization warning or exception.
• Focused regression test passes.
• Scoped analysis reports no issues.
• Debug APK builds with Flutter 3.47.5.