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/utils/diff-highlighter.test.ts

@@ -6,7 +6,6 @@ import {
   highlightDiffFile,
   parseAndHighlightDiff,
   type ParsedDiffFile,
-  type DiffHunk,
 } from "./diff-highlighter";
 
 const SIMPLE_DIFF = `diff --git a/example.ts b/example.ts