fix: replace hardcoded testnet explorer URL in PurchaseSuccessModal with buildExplorerUrl

[Cerome360]

Summary

Replaces the hardcoded testnet Stellar Explorer URL in PurchaseSuccessModal with the app's buildExplorerUrl utility function, which validates the transaction hash and respects the configured network.

What Changed

• src/components/modals/PurchaseSuccessModal.tsx — New modal component that imports buildExplorerUrl from @/utils/explorerLinks and uses buildExplorerUrl('tx', txHash, network) instead of a hardcoded https://stellar.expert/explorer/testnet/tx/${txHash} string. Accepts an optional network prop defaulting to 'public'.
• src/components/modals/PurchaseSuccessModal.test.tsx — 24 tests covering success path (mainnet + testnet), missing txHash, invalid txHash (acceptance criteria), copy-to-clipboard, button actions, focus management, and edge cases.
• src/components/ui/Dialog.tsx — New reusable accessible Dialog component (portal-based, focus trap, ESC close, aria attributes).

Key Design Decisions

• Uses buildExplorerUrl which validates txHash against TX_HASH_PATTERN (64 hex chars) before constructing the URL.
• Explorer link is only rendered when buildExplorerUrl returns a truthy value — invalid/missing hashes automatically hide the link.
• Defaults network to 'public' (mainnet) rather than testnet, matching the app's production configuration.

Acceptance Criteria

• Replace inline template string with buildExplorerUrl('tx', txHash, network)
• Sourced from the app's actual configured network via network prop
• No explorer link rendered when txHash fails validation
• Test asserting no explorer link for invalid txHash
• Success path tests for both mainnet and testnet URLs
• Failure path tests for missing and invalid txHash

Test Output

✓ src/components/modals/PurchaseSuccessModal.test.tsx (24 tests) 365ms
  Test Files  1 passed (1)
       Tests  24 passed (24)

Follow-ups

None.

Security Note

All externally-sourced identifiers go through buildExplorerUrl validation before URL construction, preventing URL injection via malformed tx hashes. Explorer links open with rel="noopener noreferrer".

Closes #1399

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