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/background/index.js

@@ -4,6 +4,7 @@
 
 const { DEFAULT_RPC_URL } = require("../shared/constants");
 const { SUPPORTED_CHAIN_IDS, networkByChainId } = require("../shared/networks");
+const { onChainSwitch } = require("../shared/chainSwitch");
 const { getBytes } = require("ethers");
 const {
     state,
@@ -345,12 +346,8 @@ async function handleRpc(method, params, origin) {
             return { result: null };
         }
         if (SUPPORTED_CHAIN_IDS.has(chainId)) {
-            // Switch to the requested network
             const target = networkByChainId(chainId);
-            state.networkId = target.id;
-            state.rpcUrl = target.defaultRpcUrl;
-            state.blockscoutUrl = target.defaultBlockscoutUrl;
-            await saveState();
+            await onChainSwitch(target.id);
             broadcastChainChanged(target.chainId);
             return { result: null };
         }

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;";
 

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 + ".");
         });
     }

src/shared/chainSwitch.js

@@ -0,0 +1,57 @@
+// Consolidated chain-switch handler.
+//
+// Every state change required when the active network changes is
+// performed here so that callers (settings UI, background
+// wallet_switchEthereumChain, future chain additions) all go
+// through a single code path.
+//
+// Adding a new chain (e.g. ETC) requires only a new entry in
+// networks.js — no per-caller wiring is needed.
+
+const { networkById } = require("./networks");
+const { clearPrices } = require("./prices");
+
+// Switch the active chain and reset all chain-specific cached state.
+// Returns the network configuration object for the new chain.
+async function onChainSwitch(newNetworkId) {
+    const { state, saveState } = require("./state");
+
+    const net = networkById(newNetworkId);
+
+    // --- core identity ---
+    state.networkId = net.id;
+    state.rpcUrl = net.defaultRpcUrl;
+    state.blockscoutUrl = net.defaultBlockscoutUrl;
+
+    // --- price cache ---
+    // Prices are chain-specific (testnet tokens are worthless,
+    // ETC has different pricing, etc.).
+    clearPrices();
+
+    // --- balance / refresh state ---
+    // Reset last-refresh timestamp so the next polling cycle
+    // triggers an immediate balance refresh on the new chain.
+    state.lastBalanceRefresh = 0;
+
+    // Clear per-address balances and token balances so stale data
+    // from the previous chain is never displayed while the first
+    // refresh on the new chain is in flight.
+    for (const wallet of state.wallets) {
+        for (const addr of wallet.addresses) {
+            addr.balance = "0";
+            addr.tokenBalances = [];
+        }
+    }
+
+    // --- chain-specific caches ---
+    // Token holder counts and fraud contract lists are
+    // chain-specific and must not carry over.
+    state.tokenHolderCache = {};
+    state.fraudContracts = [];
+
+    await saveState();
+
+    return net;
+}
+
+module.exports = { onChainSwitch };

src/shared/prices.js

@@ -8,9 +8,13 @@ const prices = {};
 let lastFetchedAt = 0;
 
 async function refreshPrices() {
-    // Testnet tokens have no real market value — skip price fetching.
+    // Testnet tokens have no real market value — skip price fetching
+    // and clear any stale mainnet prices so the UI shows no USD values.
     const { currentNetwork } = require("./state");
-    if (currentNetwork().isTestnet) return;
+    if (currentNetwork().isTestnet) {
+        clearPrices();
+        return;
+    }
     const now = Date.now();
     if (now - lastFetchedAt < PRICE_CACHE_TTL) return;
     try {
@@ -22,7 +26,19 @@ async function refreshPrices() {
     }
 }
 
+// Clear all cached prices and reset the fetch timestamp so the
+// next refreshPrices() call will fetch fresh data.
+function clearPrices() {
+    for (const key of Object.keys(prices)) {
+        delete prices[key];
+    }
+    lastFetchedAt = 0;
+}
+
+// Return the USD price for a symbol, or null on testnet / unknown.
 function getPrice(symbol) {
+    const { currentNetwork } = require("./state");
+    if (currentNetwork().isTestnet) return null;
     return prices[symbol] || null;
 }
 
@@ -40,6 +56,8 @@ function formatUsd(amount) {
 }
 
 function getAddressValueUsd(addr) {
+    const { currentNetwork } = require("./state");
+    if (currentNetwork().isTestnet) return null;
     if (!prices.ETH) return null;
     let total = 0;
     const ethBal = parseFloat(addr.balance || "0");
@@ -54,6 +72,8 @@ function getAddressValueUsd(addr) {
 }
 
 function getWalletValueUsd(wallet) {
+    const { currentNetwork } = require("./state");
+    if (currentNetwork().isTestnet) return null;
     if (!prices.ETH) return null;
     let total = 0;
     for (const addr of wallet.addresses) {
@@ -63,6 +83,8 @@ function getWalletValueUsd(wallet) {
 }
 
 function getTotalValueUsd(wallets) {
+    const { currentNetwork } = require("./state");
+    if (currentNetwork().isTestnet) return null;
     if (!prices.ETH) return null;
     let total = 0;
     for (const wallet of wallets) {
@@ -74,6 +96,7 @@ function getTotalValueUsd(wallets) {
 module.exports = {
     prices,
     refreshPrices,
+    clearPrices,
     getPrice,
     formatUsd,
     getAddressValueUsd,