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/diff-viewer.tsx

@@ -3,7 +3,7 @@ import { View, Text, ScrollView as RNScrollView } from "react-native";
 import { ScrollView as GHScrollView } from "react-native-gesture-handler";
 import { StyleSheet } from "react-native-unistyles";
 import { Fonts } from "@/constants/theme";
-import type { DiffLine, DiffSegment } from "@/utils/tool-call-parsers";
+import type { DiffLine } from "@/utils/tool-call-parsers";
 import { useWebScrollbarStyle } from "@/hooks/use-web-scrollbar-style";
 import { getCodeInsets } from "./code-insets";
 import { isWeb } from "@/constants/platform";