feat(mobile): wire quote and passkey execute into swap screen

[collinsezedike]

Summary

• Wires live Soroswap quotes into the existing swap form via lib/soroswap.getSoroswapQuote, debounced at 600 ms on amount and token changes
• Adds form | confirm | signing | submitting | done | error step states to the screen
• The confirm screen shows the quoted output, price impact, route and slippage before the user commits
• On execute: lib/soroswap.buildSoroswapSwapXdr builds the unsigned XDR, then lib/walletConnect.signXdrPayload (newly exported) simulates, signs each Soroban auth entry with the device passkey via lib/passkey.registerPasskeySigner, assembles with the fee-payer keypair, and returns the signed XDR ready to submit
• The signing state is active during the biometric prompt; submitting is active during broadcast and polling (30 x 1 s)
• All network values come from lib/network.getNetwork; wallet address from lib/walletStore.getWalletAddress
• Presentation uses the existing components/ui (Screen, Card, Button) and theme/ tokens throughout

What changed in lib/walletConnect.ts

signXdrPayload was private. Made it export so swap.tsx can drive signing independently from submission, splitting the two UI states.

Test plan

• Open swap screen: token selectors, flip button and amount input render as before
• Enter an amount: quote appears in YOU RECEIVE after 600 ms; quote card shows rate, price impact, route
• Enter an amount with no liquidity: error message in quote card; Review swap stays disabled
• Tap Review swap: confirm screen shows summary with correct values
• Tap Confirm swap: biometric sheet appears (signing state)
• Approve biometric: submitting spinner, then done screen with tx hash snippet
• Decline biometric: error state shows "Passkey authentication was declined" with Try again button
• Tap Try again: returns to form with existing inputs intact

Closes #474

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