refactor(addressbook): move from aside icon-bar to explorer nav link

Introduce generic explorer.nav_item slot type in app-main-aside.phtml
so modules can contribute links to the explorer nav panel.

Switch addressbook module from aside.tab_panel to explorer.nav_item.
Remove aside department-filter panel, AddressBookLayoutProvider, and
AddressBookSessionProvider (only served the now-removed aside panel).

Fix grid Name column overflow into Email column: CSS selector
.grid-name-cell > span:last-child never matched the <a> name link;
changed to :last-child and scoped flex-shrink:0 to :first-child only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/vendor-overrides/gridjs.css

@@ -191,12 +191,13 @@
     min-width: 0;
   }
 
-  .grid-name-cell > a {
+  .grid-name-cell > a:first-child,
+  .grid-name-cell > span:first-child {
     display: inline-flex;
     flex-shrink: 0;
   }
 
-  .grid-name-cell > span:last-child {
+  .grid-name-cell > :last-child {
     min-width: 0;
     overflow: hidden;
     text-overflow: ellipsis;