Add Base network support to PolyPay

[Huygon764]

Problem: PolyPay currently only supports Horizen network. Users cannot create or manage accounts on Base.

Expectation:

• Add global network switcher (Horizen / Base) at user wallet level in sidebar
• Switching network → auto switch wallet + display accounts of that network only
• Each network has completely independent accounts (no shared state)
• Create account flow: allow selecting 1 or 2 networks. If 2 selected → deploy 2 independent accounts with same initial config
• Backend: add chainId to Account model, add Base chain config (RPC, contract addresses), deploy verifier contract on Base
• Database migration: add chain_id column to accounts table (default = Horizen)