feat(desktop): virtualize chat thread + sidebar via TanStack Virtual

Replaces `use-stick-to-bottom` and per-row session rendering with
`@tanstack/react-virtual`, matching what Cursor uses.

Chat thread (`thread-virtualizer.tsx`):
- Natural-flow virtualization (padding spacers, not absolute items) so
  `position: sticky` on the human bubble still resolves cleanly against
  the scroller.
- Custom at-bottom anchor: pins when armed, disarms on user-driven
  upward scroll, re-arms at bottom, jumps on session switch +
  `thread.runStart`.
- Loading indicator and `--thread-last-message-clearance` move to a
  real `[data-slot=aui_composer-clearance]` node; drops the brittle
  `:nth-last-child(1 of …)` rule that can't fire reliably under
  virtualization.

Sidebar (`virtual-session-list.tsx`):
- Flat agents list virtualizes at >=25 rows; pinned and
  workspace-grouped paths stay direct-render.
- `SortableContext` keeps all IDs; only the window mounts; dnd-kit's
  `setNodeRef` is merged with `virtualizer.measureElement` so rows
  participate in both DnD hit-testing and TanStack measurement.

Drops `use-stick-to-bottom`. Streaming test gets a global
`offsetWidth/offsetHeight` stub so the virtualizer's viewport sizing
works in jsdom; the scroll-up-doesn't-pull-back invariant still passes.

apps/desktop/src/styles.css

@@ -626,21 +626,8 @@ canvas {
   }
 }
 
-/* Last thread row with a message root — avoids composer overlap (Chromium/Electron). */
-[data-slot='aui_thread-content']
-  > :nth-last-child(
-    1
-      of
-      :is(
-        [data-slot='aui_assistant-message-root'],
-        [data-slot='aui_user-message-root'],
-        [data-slot='aui_system-message-root'],
-        [data-slot='aui_edit-composer-root'],
-        [data-slot='aui_response-loading']
-      )
-  ) {
-  margin-bottom: var(--thread-last-message-clearance);
-}
+/* Bottom clearance lives on [data-slot='aui_composer-clearance'] —
+   virtualized items unmount, so :nth-last-child can't fire reliably. */
 
 [data-slot='aui_assistant-message-content'] {
   padding-left: var(--message-text-indent);