refactor: move Export Private Key into overflow menu

Replace the muted text link at the bottom of AddressDetail with a
'···' overflow/more button in the action button row. Clicking it
opens a dropdown with 'Export Private Key' as an option. Clicking
outside closes the dropdown. The pattern is reusable for future
secondary actions.

src/popup/views/addressDetail.js

@@ -264,7 +264,22 @@ function init(_ctx) {
 
     $("btn-add-token").addEventListener("click", ctx.showAddTokenView);
 
+    // More menu dropdown
+    const moreBtn = $("btn-more-menu");
+    const moreDropdown = $("more-menu-dropdown");
+    moreBtn.addEventListener("click", (e) => {
+        e.stopPropagation();
+        moreDropdown.classList.toggle("hidden");
+    });
+    document.addEventListener("click", () => {
+        moreDropdown.classList.add("hidden");
+    });
+    moreDropdown.addEventListener("click", (e) => {
+        e.stopPropagation();
+    });
+
     $("btn-export-privkey").addEventListener("click", () => {
+        moreDropdown.classList.add("hidden");
         const wallet = state.wallets[state.selectedWallet];
         const addr = wallet.addresses[state.selectedAddress];
         const title = addressTitle(addr.address, state.wallets);