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/content/inpage.js

@@ -2,7 +2,10 @@
 // Creates window.ethereum (EIP-1193 provider) and announces via EIP-6963.
 
 (function () {
-    const CHAIN_ID = "0x1"; // Ethereum mainnet
+    // Defaults to mainnet; updated dynamically via eth_chainId on init and
+    // chainChanged events from the extension.
+    let currentChainId = "0x1";
+    let currentNetworkVersion = "1";
 
     const listeners = {};
     let nextId = 1;
@@ -28,6 +31,12 @@
         if (event.source !== window) return;
         if (event.data?.type !== "AUTISTMASK_EVENT") return;
         const { eventName, data } = event.data;
+        if (eventName === "chainChanged") {
+            currentChainId = data;
+            currentNetworkVersion = String(parseInt(data, 16));
+            provider.chainId = currentChainId;
+            provider.networkVersion = currentNetworkVersion;
+        }
         emit(eventName, data);
     });
 
@@ -57,8 +66,8 @@
     const provider = {
         isAutistMask: true,
         isMetaMask: true, // compatibility — many dApps check this
-        chainId: CHAIN_ID,
-        networkVersion: "1",
+        chainId: currentChainId,
+        networkVersion: currentNetworkVersion,
         selectedAddress: null,
 
         async request(args) {
@@ -75,6 +84,12 @@
                         ? result[0]
                         : null;
             }
+            if (args.method === "eth_chainId" && result) {
+                currentChainId = result;
+                currentNetworkVersion = String(parseInt(result, 16));
+                provider.chainId = currentChainId;
+                provider.networkVersion = currentNetworkVersion;
+            }
             return result;
         },
 
@@ -189,4 +204,19 @@
 
     window.addEventListener("eip6963:requestProvider", announceProvider);
     announceProvider();
+
+    // Fetch the current chain ID from the extension on load so the provider
+    // reflects the selected network immediately (covers Sepolia etc.).
+    sendRequest({ method: "eth_chainId", params: [] })
+        .then((chainId) => {
+            if (chainId) {
+                currentChainId = chainId;
+                currentNetworkVersion = String(parseInt(chainId, 16));
+                provider.chainId = currentChainId;
+                provider.networkVersion = currentNetworkVersion;
+            }
+        })
+        .catch(() => {
+            // Best-effort — keep defaults.
+        });
 })();