feat: add Sepolia testnet support (#137)

## Summary

Adds Sepolia testnet support to AutistMask.

### Changes

- **New `src/shared/networks.js`** — centralized network definitions (mainnet + Sepolia) with chain IDs, default RPC/Blockscout endpoints, and block explorer URLs
- **State management** — `networkId` added to persisted state; defaults to mainnet for backward compatibility
- **Settings UI** — network selector dropdown lets users switch between Ethereum Mainnet and Sepolia Testnet
- **Dynamic explorer links** — all hardcoded `etherscan.io` URLs replaced with dynamic links from the current network config (`sepolia.etherscan.io` for Sepolia)
- **Background service** — `wallet_switchEthereumChain` now accepts both mainnet (0x1) and Sepolia (0xaa36a7); broadcasts `chainChanged` 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/api/v2` for Sepolia
- **Etherscan labels** — phishing/scam checks use the correct explorer per network
- **Price fetching** — skipped on testnets (testnet tokens have no real market value)
- **RPC validation** — checks against the selected network's chain ID, not hardcoded mainnet
- **ethers provider** — `getProvider()` uses the correct ethers `Network` for Sepolia

### API Endpoints Verified

| Service | Mainnet | Sepolia |
|---------|---------|--------|
| Etherscan | etherscan.io | sepolia.etherscan.io |
| Blockscout | eth.blockscout.com/api/v2 | eth-sepolia.blockscout.com/api/v2 |
| RPC | ethereum-rpc.publicnode.com | ethereum-sepolia-rpc.publicnode.com |
| CoinDesk (prices) | ✅ | N/A (skipped on testnet) |

closes #110

Reviewed-on: #137

THIS WAS ONESHOTTED USING OPUS 4.  WTAF
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

src/popup/views/transactionDetail.js

@@ -12,7 +12,7 @@ const {
     isoDate,
     timeAgo,
 } = require("./helpers");
-const { state } = require("../../shared/state");
+const { state, currentNetwork } = require("../../shared/state");
 const { formatEther, formatUnits } = require("ethers");
 const makeBlockie = require("ethereum-blockies-base64");
 const { log, debugFetch } = require("../../shared/log");
@@ -69,7 +69,7 @@ function etherscanLinkHtml(url) {
 function txAddressHtml(address, ensName, title) {
     const blockie = blockieHtml(address);
     const dot = addressDotHtml(address);
-    const link = `https://etherscan.io/address/${address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${address}`;
     const extLink = etherscanLinkHtml(link);
     let html = `<div class="mb-1">${blockie}</div>`;
     if (title) {
@@ -95,7 +95,7 @@ function txAddressHtml(address, ensName, title) {
 }
 
 function txHashHtml(hash) {
-    const link = `https://etherscan.io/tx/${hash}`;
+    const link = `${currentNetwork().explorerUrl}/tx/${hash}`;
     const extLink = etherscanLinkHtml(link);
     return copyableHtml(hash, "break-all") + extLink;
 }
@@ -172,7 +172,7 @@ function render() {
     if (tokenContractSection && tokenContractEl) {
         if (tx.contractAddress) {
             const dot = addressDotHtml(tx.contractAddress);
-            const link = `https://etherscan.io/token/${tx.contractAddress}`;
+            const link = `${currentNetwork().explorerUrl}/token/${tx.contractAddress}`;
             tokenContractEl.innerHTML =
                 `<div class="flex items-center">${dot}` +
                 copyableHtml(tx.contractAddress, "break-all") +
@@ -234,7 +234,7 @@ function showDetailField(sectionId, contentId, value) {
 function populateOnChainDetails(txData) {
     // Block number
     if (txData.block_number != null) {
-        const blockLink = `https://etherscan.io/block/${txData.block_number}`;
+        const blockLink = `${currentNetwork().explorerUrl}/block/${txData.block_number}`;
         const blockSection = $("tx-detail-block-section");
         const blockEl = $("tx-detail-block");
         if (blockSection && blockEl) {
@@ -361,12 +361,12 @@ async function loadFullTxDetails(txHash, toAddress, isContractCall) {
                     if (tokenSymbol) {
                         detailsHtml += `<div class="font-bold">${escapeHtml(tokenSymbol)}</div>`;
                     }
-                    const etherscanUrl = `https://etherscan.io/token/${d.address}`;
+                    const etherscanUrl = `${currentNetwork().explorerUrl}/token/${d.address}`;
                     detailsHtml += `<div class="flex items-center">${dot}${copyableHtml(d.address, "break-all")}${etherscanLinkHtml(etherscanUrl)}</div>`;
                 } else if (d.address) {
                     // Protocol/contract entry: show name + Etherscan link
                     const dot = addressDotHtml(d.address);
-                    const etherscanUrl = `https://etherscan.io/address/${d.address}`;
+                    const etherscanUrl = `${currentNetwork().explorerUrl}/address/${d.address}`;
                     detailsHtml += `<div class="flex items-center">${dot}${copyableHtml(d.value, "break-all")}${etherscanLinkHtml(etherscanUrl)}</div>`;
                 } else {
                     detailsHtml += `<div class="font-bold">${escapeHtml(d.value)}</div>`;