style: rework overflow menu to look like menu items, not buttons

- Menu items now use text-xs font-light for smaller, lighter type
  that's clearly distinct from the pushbuttons in the action row
- The ··· button stays visually inverted (bg-fg text-bg) while the
  dropdown menu is open, reverting when closed
- Menu items styled as plain text rows with subtle hover background,
  no borders or button-like appearance

src/popup/index.html

@@ -319,7 +319,7 @@
                         >
                             <button
                                 id="btn-export-privkey"
-                                class="block w-full text-left px-4 py-1.5 text-sm hover:bg-hover cursor-pointer"
+                                class="block w-full text-left px-4 py-1.5 text-xs font-light text-muted hover:bg-hover hover:text-fg cursor-pointer"
                             >
                                 Export Private Key
                             </button>