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/settings.js

@@ -1,7 +1,7 @@
 const { $, showView, showFlash, escapeHtml } = require("./helpers");
 const { applyTheme } = require("../theme");
-const { state, saveState } = require("../../shared/state");
-const { ETHEREUM_MAINNET_CHAIN_ID } = require("../../shared/constants");
+const { state, saveState, currentNetwork } = require("../../shared/state");
+const { NETWORKS, SUPPORTED_CHAIN_IDS } = require("../../shared/networks");
 const { log, debugFetch } = require("../../shared/log");
 const deleteWallet = require("./deleteWallet");
 
@@ -125,6 +125,10 @@ function renderWalletListSettings() {
 function show() {
     $("settings-rpc").value = state.rpcUrl;
     $("settings-blockscout").value = state.blockscoutUrl;
+    const networkSelect = $("settings-network");
+    if (networkSelect) {
+        networkSelect.value = state.networkId;
+    }
     renderTrackedTokens();
     renderSiteLists();
     renderWalletListSettings();
@@ -168,9 +172,12 @@ function init(ctx) {
                 showFlash("Endpoint returned error: " + json.error.message);
                 return;
             }
-            if (json.result !== ETHEREUM_MAINNET_CHAIN_ID) {
+            const net = currentNetwork();
+            if (json.result !== net.chainId) {
                 showFlash(
-                    "Wrong network (expected mainnet, got chain " +
+                    "Wrong network (expected " +
+                        net.name +
+                        ", got chain " +
                         json.result +
                         ").",
                 );
@@ -209,6 +216,22 @@ function init(ctx) {
         showFlash("Saved.");
     });
 
+    const networkSelect = $("settings-network");
+    if (networkSelect) {
+        networkSelect.addEventListener("change", async () => {
+            const newId = networkSelect.value;
+            const net = NETWORKS[newId];
+            if (!net) return;
+            state.networkId = newId;
+            state.rpcUrl = net.defaultRpcUrl;
+            state.blockscoutUrl = net.defaultBlockscoutUrl;
+            $("settings-rpc").value = state.rpcUrl;
+            $("settings-blockscout").value = state.blockscoutUrl;
+            await saveState();
+            showFlash("Switched to " + net.name + ".");
+        });
+    }
+
     $("settings-show-zero-balances").checked = state.showZeroBalanceTokens;
     $("settings-show-zero-balances").addEventListener("change", async () => {
         state.showZeroBalanceTokens = $("settings-show-zero-balances").checked;