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/send.js

@@ -7,7 +7,7 @@ const {
     addressTitle,
     escapeHtml,
 } = require("./helpers");
-const { state, currentAddress } = require("../../shared/state");
+const { state, currentAddress, currentNetwork } = require("../../shared/state");
 let ctx;
 const { getProvider } = require("../../shared/balances");
 const { KNOWN_SYMBOLS, resolveSymbol } = require("../../shared/tokenList");
@@ -149,7 +149,7 @@ function updateSendBalance() {
     const addr = currentAddress();
     if (!addr) return;
     const dot = addressDotHtml(addr.address);
-    const link = `https://etherscan.io/address/${addr.address}`;
+    const link = `${currentNetwork().explorerUrl}/address/${addr.address}`;
     const extLink = `<a href="${link}" target="_blank" rel="noopener" class="inline-flex items-center">${EXT_ICON}</a>`;
     const title = addressTitle(addr.address, state.wallets);
     let fromHtml = "";