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

lib/screens/main_shell.dart

@@ -891,8 +891,9 @@ class _MainShellState extends ConsumerState<MainShell>
               ],
             );
             // The backdrop blur re-filters everything scrolling underneath on
-            // every frame; low-end devices get an opaque base instead.
-            if (!ref.read(backdropBlurEnabledProvider)) {
+            // every frame; low-end devices get an opaque base instead unless
+            // the user forces blur on in appearance settings.
+            if (!ref.watch(backdropBlurEnabledProvider)) {
               return ColoredBox(
                 color: settingsGroupColor(context),
                 child: bottomBar,

lib/screens/settings/appearance_settings_page.dart

@@ -101,6 +101,15 @@ class AppearanceSettingsPage extends ConsumerWidget {
                     onChanged: (value) => ref
                         .read(settingsProvider.notifier)
                         .setHeroAnimationsEnabled(value),
+                  ),
+                  SettingsSwitchItem(
+                    icon: Icons.blur_on,
+                    title: context.l10n.appearanceForceBlur,
+                    subtitle: context.l10n.appearanceForceBlurSubtitle,
+                    value: settings.forceBackdropBlur,
+                    onChanged: (value) => ref
+                        .read(settingsProvider.notifier)
+                        .setForceBackdropBlur(value),
                     showDivider: false,
                   ),
                 ],