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/index.html

@@ -305,6 +305,26 @@
                     >
                         + Token
                     </button>
+                    <div class="relative">
+                        <button
+                            id="btn-more-menu"
+                            class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                            aria-label="More actions"
+                        >
+                            &middot;&middot;&middot;
+                        </button>
+                        <div
+                            id="more-menu-dropdown"
+                            class="hidden absolute right-0 top-full mt-1 border border-border bg-bg z-50 min-w-[10rem]"
+                        >
+                            <button
+                                id="btn-export-privkey"
+                                class="block w-full text-left px-3 py-2 text-sm hover:bg-fg hover:text-bg cursor-pointer"
+                            >
+                                Export Private Key
+                            </button>
+                        </div>
+                    </div>
                 </div>
 
                 <!-- transactions -->
@@ -316,13 +336,6 @@
                         <div class="text-muted text-xs py-1">Loading...</div>
                     </div>
                 </div>
-                <div class="mt-3 text-center">
-                    <span
-                        id="btn-export-privkey"
-                        class="text-xs text-muted underline decoration-dashed cursor-pointer hover:text-fg"
-                        >Export private key</span
-                    >
-                </div>
             </div>
 
             <!-- ============ EXPORT PRIVATE KEY VIEW ============ -->

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);