feat(AccountScreen): export account data to JSON, show Stellar thresholds collapsible section, View on Stellar Expert address link

[k-deejah]

Problem

1. AccountScreen has no export of account data to JSON or CSV — power users who want to archive their account state (balances, sequence, subentries) for auditing have no way to export it from the UI.

2. AccountScreen does not show the account thresholds (low/med/high/master weight) — Stellar multisig accounts have configurable operation thresholds. The AccountData type could expose thresholds and the AccountCard could render them in a collapsible section.

3. AccountScreen has no "View on Stellar Expert" link — the account address has no external link to block explorers, unlike TransactionPanel's success state which links to Stellar Expert.

Solution

1. Add an "Export" button in AccountScreen header that downloads account + balance data as JSON.
2. Add a thresholds?: { low: number; med: number; high: number; master: number } field to AccountData and render it in a collapsible section in AccountCard.
3. Add a "View on Stellar Expert" link below AddressDisplay in AccountCard, network-aware.

Acceptance Criteria

• Export button downloads account data as sorokit-account-{address}.json
• Thresholds section renders in AccountCard when data is available
• "View on Stellar Expert" link opens the correct network URL
• npm run build passes

---

Note for Contributors: Write a clear PR description. Include a screenshot of the thresholds section and confirm the export file is valid JSON.