Add sorting options to case browse page

[darshacharya]

Description

The case browse page only sorts by creation date (newest first). Users should be able to sort by different criteria.

Requirements

Backend

• Add a `sort` query parameter to `GET /api/cases`
• Supported values: `newest`, `oldest`, `most_funded`, `least_funded`, `urgency`

Frontend

• Add a sort dropdown next to the existing filters on `frontend/src/app/cases/page.tsx`
• Options: "Newest", "Oldest", "Most Funded", "Least Funded", "Urgency"

Tech Notes

• Backend: `backend/app/routers/cases.py` — add `order_by` logic based on the `sort` param
• For urgency sorting, map to: critical > high > medium > low

Acceptance Criteria

• Sort dropdown appears on browse page
• All 5 sort options work correctly
• Sort works in combination with category/urgency filters
• Default sort is "Newest"