feat(header): session tab tooltips, status dots, cursor context menu

Right-click now opens the session menu under the cursor (the sidebar's
context-menu pattern; the same header-supplied items back both the
"..." dropdown and the context menu via injected menu primitives) and
still never changes the active tab. Hovering a tab shows the sidebar's
session tooltip — title, last-activity time, project, branch and PR
status — after a delay, suppressed while a menu is open or a drag is in
flight. Each tab carries the sidebar's status dot at its end (accent
while the session runs, info-blue for unread), hidden while the hover
controls overlay it. Tab titles fade out instead of ending in "...",
and while the active tab is renaming its hover controls stay hidden so
only the rename controls show.

packages/ui/src/index.css

@@ -1336,6 +1336,14 @@ html:not(.dark) .chat-scroll {
   background: transparent !important;
 }
 
+/* The marked renderer uses its own code-body wrapper instead of Streamdown's. */
+.markdown-content [data-md-code-body],
+.markdown-content [data-md-code-body] pre,
+.markdown-content [data-md-code-body] code,
+.markdown-content [data-md-code-body] [data-md-code-lines] {
+  background: transparent !important;
+}
+
 .markdown-content [data-md-code-lines] {
   display: block;
   min-width: 100%;
@@ -1815,3 +1823,11 @@ html.desktop-runtime .markdown-content [data-openchamber-file-link="true"] {
   width: 0;
   height: 0;
 }
+
+/* Session tab titles: fade out instead of "..." — the ellipsis reads as
+   clutter next to the tab's status dot and hover controls. Short titles
+   never reach the fade zone (the span spans the tab, not the text). */
+.session-tab-title {
+  -webkit-mask-image: linear-gradient(to right, black calc(100% - 14px), transparent);
+  mask-image: linear-gradient(to right, black calc(100% - 14px), transparent);
+}