feat: add Sepolia testnet support

Add support for the Sepolia testnet alongside Ethereum mainnet:

- New src/shared/networks.js with network definitions (mainnet + sepolia)
  including chain IDs, default RPC/Blockscout endpoints, and explorer URLs
- State now tracks networkId; defaults to mainnet for backward compatibility
- Network selector in Settings lets users switch between mainnet and Sepolia
- Switching networks updates RPC URL, Blockscout URL, and chain ID
- All hardcoded etherscan.io URLs replaced with dynamic explorer links
  from the current network config (sepolia.etherscan.io for Sepolia)
- Background handles wallet_switchEthereumChain for both supported chains
  and broadcasts chainChanged events to connected dApps
- Inpage provider fetches chain ID on init and updates dynamically via
  chainChanged events (no more hardcoded 0x1)
- Blockscout API uses eth-sepolia.blockscout.com for Sepolia
- Etherscan label/phishing checks use the correct explorer per network
- Price fetching skipped on testnets (tokens have no real value)
- RPC validation checks against the selected network's chain ID
- getProvider() uses the correct ethers Network for Sepolia

API endpoints verified:
- Etherscan: sepolia.etherscan.io
- Blockscout: eth-sepolia.blockscout.com/api/v2
- RPC: ethereum-sepolia-rpc.publicnode.com

closes #110

src/shared/etherscanLabels.js

@@ -2,8 +2,6 @@
 // Extension users make the requests directly to Etherscan — no proxy needed.
 // This is a best-effort enrichment: network failures return null silently.
 
-const ETHERSCAN_BASE = "https://etherscan.io/address/";
-
 // Patterns in the page title that indicate a flagged address.
 // Title format: "Fake_Phishing184810 | Address: 0x... | Etherscan"
 const PHISHING_LABEL_PATTERNS = [/^Fake_Phishing/i, /^Phish:/i, /^Exploiter/i];
@@ -74,12 +72,19 @@ function parseEtherscanPage(html) {
  * Returns a warning object if the address is flagged, or null.
  * Network failures return null silently (best-effort check).
  *
+ * Uses the current network's explorer URL so the lookup works on both
+ * mainnet (etherscan.io) and Sepolia (sepolia.etherscan.io).
+ *
  * @param {string} address - Ethereum address to check.
  * @returns {Promise<{type: string, message: string, severity: string}|null>}
  */
 async function checkEtherscanLabel(address) {
     try {
-        const resp = await fetch(ETHERSCAN_BASE + address, {
+        // Lazy require to avoid pulling in chrome.storage at module scope
+        // (which breaks unit tests that only exercise parseEtherscanPage).
+        const { currentNetwork } = require("./state");
+        const etherscanBase = currentNetwork().explorerUrl + "/address/";
+        const resp = await fetch(etherscanBase + address, {
             headers: { Accept: "text/html" },
         });
         if (!resp.ok) return null;