feat(mobile): implement vault management screens — create vault, deposit USDC, and withdraw flow

[0xDeon]

Overview

Core vault operations are not implemented in the Flutter mobile app. Users need to create vaults, deposit USDC, and withdraw on mobile, matching the functionality of the web DApp.

Requirements

• Vault list screen: shows all user vaults with balance, APY, and a create button
• Vault detail screen: allocation breakdown, performance chart (last 30 days), deposit and withdraw buttons
• Deposit flow: amount input → Stellar fee preview → Freighter/mobile wallet signing → confirmation
• Withdraw flow: shares input or USDC amount → preview (gross and net of fees) → wallet signing → confirmation
• Create vault flow: simplified version of the DApp wizard (name + protocol selection, no AI step on mobile initially)
• All screens pull from the Go API and trigger Soroban contract calls via the Stellar mobile SDK

Acceptance Criteria

• Vault list screen lists all user vaults with key metrics
• Vault detail shows allocations and 30-day performance chart
• Deposit flow completes a real on-chain deposit (testnet)
• Withdraw flow uses preview_withdraw_net for accurate amount display
• Create vault flow creates a vault via the Go API

Reference: Mobile App — Vault management screens in PRD