feat(appearance): manual toggle to force backdrop blur on lower device tiers

lib/providers/runtime_profile_provider.dart

@@ -1,10 +1,19 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:spotiflac_android/providers/settings_provider.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);
 
-/// Generic visual capability decided by the startup runtime profile. Effects
-/// can consume this without hardcoding a device model or platform source.
-final backdropBlurEnabledProvider = Provider<bool>((ref) => false);
+/// Raw device capability decided by the startup runtime profile (overridden
+/// in main.dart). Only the `high` tier enables blur by default.
+final deviceSupportsBackdropBlurProvider = Provider<bool>((ref) => false);
+
+/// Whether backdrop blur effects should render: the device default, or the
+/// user's manual override from appearance settings (issue #488 — let lower
+/// tiers opt back in).
+final backdropBlurEnabledProvider = Provider<bool>((ref) {
+  return ref.watch(deviceSupportsBackdropBlurProvider) ||
+      ref.watch(settingsProvider.select((s) => s.forceBackdropBlur));
+});

lib/providers/settings_provider.dart

@@ -658,6 +658,11 @@ class SettingsNotifier extends Notifier<AppSettings> {
     _saveSettings();
   }
 
+  void setForceBackdropBlur(bool enabled) {
+    state = state.copyWith(forceBackdropBlur: enabled);
+    _saveSettings();
+  }
+
   void setExtensionVerificationBrowserMode(String mode) {
     state = state.copyWith(
       extensionVerificationBrowserMode: