KataglyphisAppShell<T> class

The application shell both Kataglyphis Flutter apps are built on.

It owns everything the two main.dart files used to duplicate line for line: the AnimationController/CurvedAnimation pair, the width-breakpoint block in didChangeDependencies, the four handle* callbacks, the theme pair, the localization delegate list and the FutureBuilder -> AppAttributes -> RoutesCreator -> MaterialApp.router tail. The apps keep only what genuinely differs: how they load their settings, and which app-owned types those settings turn into.

Three inconsistencies between the two copies were resolved here, once:

• AppSettings.supportedLocales is null-checked rather than forced with !. AppSettings.fromJsonFile defaults a missing supportedLocales key to the empty list, so the bang was never the crash it looked like - it was an empty supportedLocales handed to MaterialApp and a RangeError one line later. fallbackLocales covers that case instead.
• MaterialApp.locale follows the selected language index. Pinning it to the first supported locale (as OmniAccelerANT did) left the visible UI switching - Localizations.override inside this package's pages still worked - while onGenerateTitle stayed stuck on the first language, which made the language menu look half broken.
• The failure screen is wrapped in a MaterialApp. A bare Material subtree has no Directionality above it at this point in the tree, so rendering one throws while trying to report the original error.