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/web/server/lib/opencode/settings-registry.json

@@ -682,6 +682,10 @@
       "scope": "device",
       "local": true
     },
+    "alwaysShowScrollbars": {
+      "scope": "device",
+      "local": true
+    },
     "agentMemoryViewedAt": {
       "scope": "device",
       "local": true