Add ENS support: reverse lookup and forward resolution

Reverse ENS lookup on balance refresh — if an address has an
ENS name, it's shown in the wallet list and address detail view.
Send form accepts ENS names in the To field (resolves before
sending). Placeholder updated to indicate ENS support.

src/popup/index.html

@@ -166,6 +166,7 @@
                     </button>
                 </div>
 
+                <div id="address-ens" class="font-bold mb-1 hidden"></div>
                 <div
                     id="address-full"
                     class="text-xs break-all mb-1 cursor-pointer"
@@ -242,7 +243,7 @@
                         type="text"
                         id="send-to"
                         class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
-                        placeholder="Recipient address (0x...)"
+                        placeholder="Address (0x...) or ENS name"
                     />
                 </div>
                 <div class="mb-2">