fix(overview): center empty state messages and disable scroll when list is empty

Add ov-empty CSS class to .ov-scroll-area when an overview list has no
items — disables overflow scrolling and uses flexbox to vertically and
horizontally center the empty message. Class is removed automatically
when data is present. Keeps the normal scrollable layout intact for
populated lists.

extension/dashboard/pages/overview.js

@@ -18,6 +18,7 @@ class OvList {
     this.sentinel   = $(sentinelId);
     this.searchEl   = $(searchId);
     this.subtitleEl = $(subtitleId);
+    this.scrollArea = this.body?.closest('.ov-scroll-area') || null;
     this.rowFn      = rowFn;
     this.emptyMsg   = emptyMsg;
     this.cols       = cols;
@@ -66,8 +67,10 @@ class OvList {
     this.body.innerHTML = '';
     if (this.filtered.length === 0) {
       this.body.innerHTML = `<tr><td colspan="${this.cols}" class="empty">${this.emptyMsg}</td></tr>`;
+      this.scrollArea?.classList.add('ov-empty');
       return;
     }
+    this.scrollArea?.classList.remove('ov-empty');
     this._appendPage();
   }