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/background/index.js

@@ -2,12 +2,15 @@
 // Handles EIP-1193 RPC requests from content scripts and proxies
 // non-sensitive calls to the configured Ethereum JSON-RPC endpoint.
 
-const {
-    ETHEREUM_MAINNET_CHAIN_ID,
-    DEFAULT_RPC_URL,
-} = require("../shared/constants");
+const { DEFAULT_RPC_URL } = require("../shared/constants");
+const { SUPPORTED_CHAIN_IDS, networkByChainId } = require("../shared/networks");
 const { getBytes } = require("ethers");
-const { state, loadState, saveState } = require("../shared/state");
+const {
+    state,
+    loadState,
+    saveState,
+    currentNetwork,
+} = require("../shared/state");
 const { refreshBalances, getProvider } = require("../shared/balances");
 const { debugFetch } = require("../shared/log");
 const { decryptWithPassword } = require("../shared/vault");
@@ -329,31 +332,47 @@ async function handleRpc(method, params, origin) {
     }
 
     if (method === "eth_chainId") {
-        return { result: ETHEREUM_MAINNET_CHAIN_ID };
+        return { result: currentNetwork().chainId };
     }
 
     if (method === "net_version") {
-        return { result: "1" };
+        return { result: currentNetwork().networkVersion };
     }
 
     if (method === "wallet_switchEthereumChain") {
         const chainId = params?.[0]?.chainId;
-        if (chainId === ETHEREUM_MAINNET_CHAIN_ID) {
+        if (chainId === currentNetwork().chainId) {
+            return { result: null };
+        }
+        if (SUPPORTED_CHAIN_IDS.has(chainId)) {
+            // Switch to the requested network
+            const target = networkByChainId(chainId);
+            state.networkId = target.id;
+            state.rpcUrl = target.defaultRpcUrl;
+            state.blockscoutUrl = target.defaultBlockscoutUrl;
+            await saveState();
+            broadcastChainChanged(target.chainId);
             return { result: null };
         }
         return {
             error: {
                 code: 4902,
-                message: "AutistMask only supports Ethereum mainnet.",
+                message:
+                    "AutistMask supports Ethereum Mainnet and Sepolia Testnet only.",
             },
         };
     }
 
     if (method === "wallet_addEthereumChain") {
+        const chainId = params?.[0]?.chainId;
+        if (SUPPORTED_CHAIN_IDS.has(chainId)) {
+            return { result: null };
+        }
         return {
             error: {
                 code: 4902,
-                message: "AutistMask only supports Ethereum mainnet.",
+                message:
+                    "AutistMask supports Ethereum Mainnet and Sepolia Testnet only.",
             },
         };
     }
@@ -499,6 +518,27 @@ async function handleRpc(method, params, origin) {
     return { error: { message: "Unsupported method: " + method } };
 }
 
+// Broadcast chainChanged to all tabs when the network is switched.
+function broadcastChainChanged(chainId) {
+    tabsApi.query({}, (tabs) => {
+        for (const tab of tabs) {
+            tabsApi.sendMessage(
+                tab.id,
+                {
+                    type: "AUTISTMASK_EVENT",
+                    eventName: "chainChanged",
+                    data: chainId,
+                },
+                () => {
+                    if (runtime.lastError) {
+                        // expected for tabs without our content script
+                    }
+                },
+            );
+        }
+    });
+}
+
 // Broadcast accountsChanged to all tabs, respecting per-address permissions
 async function broadcastAccountsChanged() {
     // Clear non-remembered approvals on address switch