fix: switching the network in Settings tells open pages (closes #500)

Once Settings has saved the new network it asks the background to send
chainChanged with the new chain id to every open tab, through the same
function an approved site switch request uses. Only the extension's own
pages can ask for that. Choosing the network already active changes
nothing and sends nothing.

Model: opus-5-5

src/popup/views/settings.js

@@ -316,7 +316,11 @@ function init(ctx) {
     const networkSelect = $("settings-network");
     networkSelect.addEventListener("change", async () => {
         const newId = networkSelect.value;
+        if (newId === state.networkId) return;
         const net = await onChainSwitch(newId);
+        // Open pages are told by the background, as after a site's approved
+        // switch request.
+        notify({ type: "AUTISTMASK_NETWORK_CHANGED", chainId: net.chainId });
         $("settings-rpc").value = state.rpcUrl;
         $("settings-blockscout").value = state.blockscoutUrl;
         showFlash("Switched to " + net.name + ".");