fix(header): one session-tab menu, hidden scrollbar, right-click, control order

The strip now owns a single dropdown per tab, fed by the header with
items bound to that tab's session — rename (activates the tab first),
copy id, share/copy link/unshare, export and move-to-worktree (active
tab only, they need the loaded directory), close other tabs, archive
and delete with the confirm dialog targeting the right session. The
separate inactive-tab menu is gone, and there is no Close item — the
tab's close button covers it, now placed after the menu button.

Right-click opens that menu without activating the tab. The menu's
anchor overlay stays mounted until the close animation finishes, which
removes the popup flashing in the top-left corner on close. The
scroller hides its scrollbar via a dedicated CSS class (the bar was
shifting the header content vertically).

packages/ui/src/index.css

@@ -1803,3 +1803,15 @@ html.desktop-runtime .markdown-content [data-openchamber-file-link="true"] {
 .session-tab-slot:not([data-active='true']):hover + .session-tab-slot::before {
   display: none;
 }
+
+/* Header session tabs scroller: never show a scrollbar (it shifts the header
+   content vertically); overflow is communicated by the edge fades. */
+.session-tabs-scroll {
+  scrollbar-width: none;
+  -ms-overflow-style: none;
+}
+.session-tabs-scroll::-webkit-scrollbar {
+  display: none;
+  width: 0;
+  height: 0;
+}