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/settings.js

@@ -2,6 +2,7 @@ const { $, showView, showFlash, escapeHtml } = require("./helpers");
 const { applyTheme } = require("../theme");
 const { state, saveState, currentNetwork } = require("../../shared/state");
 const { NETWORKS, SUPPORTED_CHAIN_IDS } = require("../../shared/networks");
+const { onChainSwitch } = require("../../shared/chainSwitch");
 const { log, debugFetch } = require("../../shared/log");
 const deleteWallet = require("./deleteWallet");
 
@@ -220,14 +221,9 @@ function init(ctx) {
     if (networkSelect) {
         networkSelect.addEventListener("change", async () => {
             const newId = networkSelect.value;
-            const net = NETWORKS[newId];
-            if (!net) return;
-            state.networkId = newId;
-            state.rpcUrl = net.defaultRpcUrl;
-            state.blockscoutUrl = net.defaultBlockscoutUrl;
+            const net = await onChainSwitch(newId);
             $("settings-rpc").value = state.rpcUrl;
             $("settings-blockscout").value = state.blockscoutUrl;
-            await saveState();
             showFlash("Switched to " + net.name + ".");
         });
     }