Introduces src/shared/chainSwitch.js with a single onChainSwitch()
function that handles every state change required when switching
networks:
- Updates networkId, rpcUrl, blockscoutUrl from network config
- Clears price caches (testnet tokens are worthless)
- Resets lastBalanceRefresh to force immediate refresh
- Clears per-address balances and token balances
- Clears tokenHolderCache and fraudContracts (chain-specific)
- Persists state
Both callers (settings UI dropdown, background
wallet_switchEthereumChain) now go through this single code path.
Adding a new chain (e.g. ETC) requires only a new entry in
networks.js with no per-caller wiring.
Also adds defense-in-depth testnet checks in prices.js (getPrice,
getAddressValueUsd, etc.) and fixes a hardcoded etherscan.io link
in addressToken.js to use the network-aware explorer URL.
Closes#139
