fix: add transaction explorer open action

[ghg001]

Overview

This PR adds an Explorer Open Action to the transaction detail screen. Users can now open a transaction hash in a Stellar Testnet explorer directly from the UI via the platform linking API. The action is disabled for invalid or missing hashes, handles linking failures with a clear user-facing message, and the interface makes the Testnet context explicit to avoid confusion with Mainnet.

Related Issue

Closes #

Changes

🔍 Transaction Explorer Action

• [ADD] src/services/stellar.ts

  • Adds getTestnetTransactionUrl(txHash) helper that returns a Stellar Testnet transaction URL.
  • Uses https://stellar.expert/explorer/testnet/tx/... and avoids Mainnet URLs.
  • Returns null for missing or malformed hashes so callers can disable the action safely.

• [MODIFY] app/transaction/[id].tsx

  • Adds the explorer action to the transaction detail screen.
  • Calls Linking.openURL with the Testnet URL and handles rejection with a clear alert.
  • Disables the action when the transaction hash is unavailable or invalid.

• [MODIFY] src/features/transactions/helpers.ts

  • Adds isValidTransactionHash and normalizeTransactionHash utilities used before opening the URL.

• [MODIFY] src/features/settings/useNetworkEnvironment.ts

  • Exposes the current network environment so the action is used only in Testnet contexts.

• [MODIFY] src/components/Button.tsx

  • Adds disabled handling and visual styling so the explorer action cannot be pressed when the hash is invalid.

Verification Results

npm test
✅ 12/12 passed

Live acceptance check:
✅ Testnet URL generated with correct stellar.expert testnet path
✅ Invalid or missing hash disables the action
✅ Linking failure shows a clear alert
✅ Testnet label is visible in the transaction detail UI

Acceptance Criteria	Status
Transaction detail screen includes an explorer action	✅ Added in app/transaction/[id].tsx
Explorer action opens a Testnet transaction URL	✅ Uses getTestnetTransactionUrl in src/services/stellar.ts
Invalid or missing transaction hash disables the action	✅ isValidTransactionHash gate + Button disabled state
Linking failure shows a clear message	✅ Alert shown on Linking.openURL rejection
The UI makes the Testnet context clear	✅ Button label and helper copy both mention Testnet

Closes #97