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/addressToken.js

@@ -12,7 +12,12 @@ const {
     truncateMiddle,
     balanceLine,
 } = require("./helpers");
-const { state, currentAddress, saveState } = require("../../shared/state");
+const {
+    state,
+    currentAddress,
+    saveState,
+    currentNetwork,
+} = require("../../shared/state");
 const { TOKEN_BY_ADDRESS, resolveSymbol } = require("../../shared/tokenList");
 const {
     formatUsd,
@@ -42,7 +47,7 @@ const EXT_ICON =
     `</svg></span>`;
 
 function etherscanAddressLink(address) {
-    return `https://etherscan.io/address/${address}`;
+    return `${currentNetwork().explorerUrl}/address/${address}`;
 }
 
 function isoDate(timestamp) {
@@ -194,7 +199,7 @@ function show() {
                     : null;
         const tokenHolders = tb && tb.holders != null ? tb.holders : null;
         const dot = addressDotHtml(tokenId);
-        const tokenLink = `https://etherscan.io/token/${escapeHtml(tokenId)}`;
+        const tokenLink = `${currentNetwork().explorerUrl}/token/${escapeHtml(tokenId)}`;
         const projectUrl = knownToken && knownToken.url ? knownToken.url : null;
         let infoHtml = `<div class="font-bold mb-2">Contract Address</div>`;
         infoHtml +=
@@ -381,7 +386,7 @@ function init(_ctx) {
         let staticHtml = `<div class="font-bold">${escapeHtml(currentSymbol)}</div>`;
         if (tokenId !== "ETH") {
             const dot = addressDotHtml(tokenId);
-            const link = `https://etherscan.io/token/${tokenId}`;
+            const link = `${currentNetwork().explorerUrl}/token/${tokenId}`;
             const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
             staticHtml +=
                 `<div class="flex items-center text-xs">${dot}` +