Build case and jurisdiction explorer routes

[awitherow]

Goal

Build /jurisdictions, /jurisdictions/[id], /cases, and /cases/[id] routes over JSON data.

Acceptance criteria

• Case list can filter by jurisdiction, flag, status, and source confidence.
• Case detail explains why the case is flagged.
• Source receipts are visible beside every claim.
• Seed/unverified claims are visually distinct from official records.
• pnpm build passes.