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/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 };
         }