Clear scanning flash when address scan completes

The "Scanning for addresses..." flash had a 30-second timeout and was
never cleared on completion. Now shows "Found N addresses." when
multiple are found, or briefly shows "Ready." for single-address
wallets.

src/popup/views/addWallet.js

@@ -103,6 +103,9 @@ function init(ctx) {
             wallet.nextIndex = scan.nextIndex;
             await saveState();
             ctx.renderWalletList();
+            showFlash("Found " + scan.addresses.length + " addresses.");
+        } else {
+            showFlash("Ready.", 1000);
         }
 
         ctx.doRefreshAndRender();