fix: consolidate chain-switching into onChainSwitch()

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

src/popup/views/addressToken.js

@@ -51,7 +51,7 @@ function etherscanAddressLink(address) {
 }
 
 function etherscanTokenLink(tokenContract, holderAddress) {
-    return `https://etherscan.io/token/${tokenContract}?a=${holderAddress}`;
+    return `${currentNetwork().explorerUrl}/token/${tokenContract}?a=${holderAddress}`;
 }
 
 function isoDate(timestamp) {
@@ -168,7 +168,7 @@ function show() {
         `<a href="${addrLink}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
 
     // USD total for this token only
-    const usdVal = price ? amount * price : 0;
+    const usdVal = price ? amount * price : null;
     const usdStr = formatUsd(usdVal);
     $("address-token-usd-total").innerHTML = usdStr || "&nbsp;";