feat: add transaction tracking

[Mosas2000]

Summary

Adds reusable in-flight transaction tracking for deposit and withdrawal flows, plus the API endpoint and portfolio state needed to surface pending activity.

Changes

• Keep deposit/withdraw modals in a processing state after submission instead of closing immediately.
• Show the Stellar transaction hash and explorer link while the transaction is pending.
• Add a transaction lookup endpoint for on-chain status resolution.
• Mark portfolio transactions as pending first, then surface them in a dedicated pending section.
• Preserve the existing confirmation flow once the transaction settles.

Notes

• This branch contains only the frontend/API transaction-tracking work.

Closes #213

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