perf(ui): skip the shell backdrop blur on low-end devices

The sigma-18 BackdropFilter re-filters everything scrolling underneath
every frame. The startup runtime profile now feeds a lowEndDevice
provider; those devices get an opaque bottom bar instead.

lib/providers/runtime_profile_provider.dart

@@ -0,0 +1,6 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// True on low-end hardware (arm32-only or low-RAM, resolved at startup in
+/// main.dart via a ProviderScope override). UI uses it to skip expensive
+/// effects like the shell's backdrop blur.
+final lowEndDeviceProvider = Provider<bool>((ref) => false);