feat(TransactionPanel): explorer-linked tx hash, asset selector, memo…

[taiwoMoil]

closes #174

Description

Fixes three UX/safety gaps in the TransactionPanel send flow:

1. Explorer-linked transaction hash — the success-state hash is now a
   network-aware link to Stellar Expert
   (https://stellar.expert/explorer/{public|testnet}/tx/{hash}), with a
   "View on Stellar Expert" link. Networks Stellar Expert doesn't index
   (futurenet/localnet) fall back to plain text.
2. Asset selector — replaced the hardcoded asset: "XLM" with an Asset
   <select> populated from the connected wallet's balances (from context).
   The amount label tracks the selected asset (Amount (USDC), etc.) and falls
   back to XLM when no balances are loaded.
3. Memo type selector — added a Memo Type selector (Text / ID / None).
   memoType is passed to transaction.submit; None omits the memo entirely
   and ID validates the value is an unsigned integer before allowing submit.

Note: the working TransactionPanel implementation had been replaced with a
JSDoc-only stub in 0eeba22. This PR restores it from 0eeba22~1 and builds
the three features on top, so the diff includes that restoration.

The design-system components this panel relies on (Button, cn) require runtime
deps that had been dropped from package.json; re-added so the library builds:
@radix-ui/react-slot, clsx, tailwind-merge.

Related Issues

Closes #174

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Testing

Describe how you tested your changes:

• Component renders correctly
• Tests added or updated for this change
• No console errors or warnings
• Lint passes (npm run lint) — TransactionPanel.tsx passes with zero
  findings. (Repo-wide lint has pre-existing errors in untouched files unrelated
  to this change.)
• Build succeeds (npm run build)
• Manual testing completed

Checklist

• My code follows the style guidelines of this project
• I have performed a self-rev

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