feat: tx lookup, live rates, fee USD preview, address book (#259 #260 #262 #263)

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Summary

Closes #259
Closes #260
Closes #392
Closes #263

Changes

#259 — Transaction detail deep-link

• New TxLookup modal: enter a hash (+ public key if no account loaded) to fetch and display full transaction details
• Auto-opens when the URL contains #tx= (shareable deep-link)
• 🔍 button in the header for manual lookup

#260 — Address book

• AddressBook component wired into the payment form above the recipient field
• Contacts persist to localStorage; clicking "Use" auto-fills the recipient input
• Current recipient address is pre-filled when opening the "Add Contact" form

#262 — Live exchange rate in payment form

• New useExchangeRate hook fetches GET /api/stellar/exchange-rate/XLM/USD on mount
• Updates in real-time when a rateChange WebSocket event arrives (subscribes to the rates channel)
• Displays ≈ $X.XX USD · live rate below the amount input when a valid amount is entered
• Fixed WS envelope unwrapping ({ data, sig } → inner payload) that was silently breaking all broadcast message handling

#263 — Fee estimate USD total

• FeeDisplay now shows the total cost (amount + fee) in USD alongside XLM, using the xlmUsd rate already returned by /api/stellar/fee-stats

Testing

• No new test failures introduced (pre-existing failures unchanged)
• Build verified with vite build

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