fix: make the new-chat + menu visible and clickable

The menu opened but never appeared: it was absolutely positioned inside
.copilot-tabs, whose overflow-x: auto clips on both axes, so the dropdown
was cut off inside the tab strip. And once visible, every click would have
landed on the transparent full-screen overlay (z-index 99) above the menu
(z-index 20), closing it instead of choosing an entry.

Anchor the menu to the + button with fixed positioning (the same escape
the model dropdown gets from living outside any clipping container) and
raise it to z-index 100, above the overlay it shares with the other pills.

web/css/copilot.css

@@ -280,10 +280,8 @@ app-copilot[mode="full"] .copilot-msg {
 }
 
 .copilot-tab-menu {
-  position: absolute;
-  top: 100%;
-  left: 0;
-  z-index: 20;
+  position: fixed;
+  z-index: 100;
   min-width: 180px;
   max-height: 260px;
   overflow-y: auto;