feat(ui): add setting to keep the action bar visible while scrolling (#1095)

Adds a 'Keep Action Bar Visible' toggle (default off) under
Settings > Interface > Layout Settings. When enabled, the controls bar
(Refresh, Download, etc.) and the breadcrumb nav are wrapped in a shared
sticky container (.sticky-topbar) so both stay pinned as one unit; when
disabled, the wrapper is display: contents and the original behavior
(only the breadcrumb stays visible) is preserved.

static/js/state/index.js

@@ -59,6 +59,7 @@ const DEFAULT_SETTINGS_BASE = Object.freeze({
     strip_lora_on_copy: false,
     use_new_license_icons: true,
     group_by_model: false,
+    sticky_controls: false,
     llm_provider: 'openai',
     llm_api_key: '',
     llm_api_base: '',