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/popup/views/home.js

@@ -11,7 +11,12 @@ const {
     escapeHtml,
     truncateMiddle,
 } = require("./helpers");
-const { state, saveState, currentAddress } = require("../../shared/state");
+const {
+    state,
+    saveState,
+    currentAddress,
+    currentNetwork,
+} = require("../../shared/state");
 const {
     updateSendBalance,
     renderSendTokenSelect,
@@ -82,7 +87,7 @@ function renderActiveAddress() {
     if (state.activeAddress) {
         const addr = state.activeAddress;
         const dot = addressDotHtml(addr);
-        const link = `https://etherscan.io/address/${addr}`;
+        const link = `${currentNetwork().explorerUrl}/address/${addr}`;
         el.innerHTML =
             `<span class="underline decoration-dashed cursor-pointer" id="active-addr-copy">${dot}${escapeHtml(addr)}</span>` +
             `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;