fix(#994,#996): replace undefined CSS vars; guard overflow rebuild when open

dashboard.css used var(--surface0/1/2) and var(--text) throughout the
agent context menu, tabbar overflow dropdown, and logs toolbar — none
of which are declared in base.css (the palette only defines --bg-elev,
--border, --purple-dim, --fg etc). This caused all three dropdowns to
render with a transparent background (issue #996).

Replacements:
  --surface0  → --bg-elev   (dropdown background)
  --surface1  → --border    (hover / active state)
  --surface2  → --purple-dim (border / separator)
  --text      → --fg        (label colour)

Also addresses two argus yellows from the PR #997 review:
- Skip updateTabbarOverflow() early when the dropdown is open; avoids
  DOM flicker from the 1s badge-update tick while the menu is visible.
- Fall back to 32px when overflowBtn.offsetWidth === 0 (hidden on the
  first render) so the initial width measurement is not off.

frontend/packages/dashboard/src/tabs.js

@@ -3438,6 +3438,9 @@ window.marked = marked;
   function updateTabbarOverflow() {
     const tabbar = $('tabbar');
     if (!tabbar || !overflowBtn || !overflowDrop) return;
+    // Skip dropdown rebuild while it is open — the 1s badge tick would
+    // replace DOM nodes and cause a flicker mid-interaction.
+    if (overflowOpen) return;
 
     // Collect all tabs that are not JS-hidden (P33RS/M4TR1X may be hidden
     // by feature-gating) and not already removed from the DOM.
@@ -3452,7 +3455,10 @@ window.marked = marked;
 
     // Step 2: measure available width (bar width minus overflow-btn width).
     // We must read layout AFTER restoring all dynamic tabs.
-    const availWidth = tabbar.clientWidth - overflowBtn.offsetWidth - 8;
+    // overflowBtn may be hidden (offsetWidth=0) on the very first call; fall
+    // back to a 32px estimate so the first-pass measurement isn't off.
+    const btnW = overflowBtn.offsetWidth || 32;
+    const availWidth = tabbar.clientWidth - btnW - 8;
 
     // Step 3: walk dynamic tabs left-to-right, accumulating widths.
     // Any that go over the available width get overflowed.