Surface the live testnet contracts as first-class entry points in the explorer

[priscaenoch]

Problem

The Octraban contracts (the explorer registry and the ticket contract) are deployed to testnet, but the UI provides no direct way to open them. Users must already know a contract ID to inspect anything.

What needs to be done

• Add a small, configurable registry of "known Octraban contracts" (id, label, description) sourced from config/constants, seeded with the deployed testnet IDs.
• Show them on the explorer home (and/or the landing page) as clickable entries that route to /contract/:id.
• Each entry links out to stellar.expert as a secondary action.
• Keep the list config-driven so mainnet IDs can be added later without code changes.

Files

• new src/config/knownContracts.ts (or similar)
• src/pages/Home.tsx
• src/pages/ContractPage.tsx (ensure it renders for these IDs against the configured network)

Acceptance deliverables

• The explorer home shows the deployed registry and ticket contracts as entry points.
• Clicking one opens the contract detail page for that ID.

Tests to pass

• Test: the known-contracts list renders and each entry links to /contract/<id>.