chore: apply oxlint fix-suggestions (unused imports + useless fragments)

Scoped to only `no-unused-vars` and `react/jsx-no-useless-fragment` —
the safe subset of --fix-suggestions. Primary changes:

- Remove unused named imports from statements
- Remove unused default/type imports entirely
- Collapse `<><Single/></>` fragments to `<Single/>`

Verified clean: typecheck + format + lint all green.

packages/app/src/components/agent-form/agent-form-dropdowns.tsx

@@ -28,7 +28,7 @@ import type {
 } from "@server/server/agent/agent-sdk-types";
 import type { AgentProviderDefinition } from "@server/server/agent/provider-manifest";
 import { getModeVisuals, type AgentModeIcon } from "@server/server/agent/provider-manifest";
-import { Combobox, ComboboxItem, ComboboxEmpty } from "@/components/ui/combobox";
+import { Combobox, ComboboxItem } from "@/components/ui/combobox";
 import {
   IsolatedBottomSheetModal,
   useIsolatedBottomSheetVisibility,