feat: add an always-show scrollbars preference

Adds a device-local setting to keep overlay scrollbars visible.
Surfaces the setting in visual settings and settings search.
Updates scrollbar behavior and tests for the new preference.

packages/ui/src/lib/settings/registry.ts

@@ -544,6 +544,7 @@ export const LOCAL_DEVICE_KEYS = [
   'linearIssueListPriority',
   'showTerminalQuickKeysOnDesktop',
   'dockBadgeEnabled',
+  'alwaysShowScrollbars',
   'agentMemoryViewedAt',
   'projectContextSidebarWidth',
 ] as const;