feat(ui): add session sidebar search and bulk selection

Adds an optional session filter bar to the left sidebar with title search across parent/child sessions and a scoped Select All. Introduces multi-select checkboxes, bulk delete with clear selection controls, and confirmation dialogs for both single and bulk deletions using the existing alert dialog flow. Updates session i18n strings across supported locales.

packages/ui/src/components/instance/instance-shell2.tsx

@@ -911,6 +911,7 @@ const InstanceShell2: Component<InstanceShellProps> = (props) => {
               void result.catch((error) => log.error("Failed to create session:", error))
             }
           }}
+          enableFilterBar
           showHeader={false}
           showFooter={false}
         />