Fix/404 detail not found errors

[chuks68]

Closes #131
This PR improves error-handling UX for missing resources in details panels. Previously, all asynchronous error states were rendered as flat red messages using state.message, which did not distinguish a 404 Not Found response from a 500 Server Error or a network failure.

Changes
src/hooks/useAsync.ts: Updated the custom hook to store and expose the original error object (state.error?: unknown) under the async state so components can inspect details beyond just the stringified message.
src/components/AnchorDetail.tsx: Checks if the async loader error is an ApiRequestError with status code 404 and displays a distinct "Anchor not found" message with a link back to /anchors instead of the generic red text.
src/components/SettlementDetail.tsx: Applies identical logic to render a distinct "Settlement not found" message with a link back to /settlements.
src/components/AnchorDetail.test.tsx: Added a mock unit test covering the 404 error branch for the anchor details loader.
src/components/SettlementDetail.test.tsx: Added a mock unit test covering the 404 error branch for the settlement details loader.

[Jagadeeshftw]

Thanks — after resolving the merge, this ends up fully redundant with main: the PoolsPanel memoization and SettlementTable responsive card-layout content already landed via #349/#352, and the 404 detail-page messaging commit here matches what's already on main too. Zero net diff left, so closing as already covered rather than merging a duplicate.