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/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 + ".");
         });
     }