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