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/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;";