fix: suppress USD display on testnet networks (#142)

## Summary

Fixes USD prices still showing on the main view when connected to a testnet (e.g. Sepolia). The root cause was stale mainnet prices lingering in the in-memory price cache after switching networks.

### Root Cause

PR #137 correctly made `refreshPrices()` skip fetching on testnets, but the cached prices from a prior mainnet session remained in the `prices` object. All display functions (`getPrice()`, `getAddressValueUsd()`, etc.) used whatever was cached without checking which network was active.

### Changes

- **`src/shared/prices.js`**
  - `refreshPrices()` now clears the price cache when on a testnet instead of silently returning
  - New `clearPrices()` function empties the cache and resets the fetch timestamp
  - `getPrice()` returns null on testnets (defense-in-depth)
  - `getAddressValueUsd()`, `getWalletValueUsd()`, `getTotalValueUsd()` return null on testnets

- **`src/popup/views/settings.js`**
  - Network switcher immediately clears prices when switching to a testnet, so the UI updates without waiting for the next refresh cycle

closes #139

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #142
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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 };