fix(desktop): keep pinned + recent sessions visible across compression

Long-running sessions auto-compress: the gateway ends the original session
and surfaces the live continuation under a new id (list_sessions_rich projects
the root forward to its tip). Two symptoms fell out of the id rotation:

- A pinned session "vanished" — the pin is stored as the pre-compression root
  id, but the sidebar only matched on the live id, so it was filtered out.
  Pins now resolve on the durable lineage-root id (`_lineage_root_id`, already
  surfaced by the projection): the sidebar indexes sessions by both ids, pin/
  unpin and reorder operate on the durable id, and `sessionPinId()` is shared
  with the Cmd+P toggle. Existing pins keep working with no migration.

- A freshly-continued session was missing from the list until you ungrouped +
  "load 50 more" — the list paginated by original start time, so an old-but-
  active conversation sat past the first page. The desktop now requests
  `order=recent` (GET /api/sessions gains an `order` param backed by the
  existing recency CTE), surfacing live continuations on the first page.

apps/desktop/src/app/desktop-controller.tsx

@@ -32,6 +32,8 @@ import {
   $freshDraftReady,
   $gatewayState,
   $selectedStoredSessionId,
+  $sessions,
+  sessionPinId,
   setAwaitingResponse,
   setBusy,
   setCurrentBranch,
@@ -224,10 +226,14 @@ export function DesktopController() {
       return
     }
 
-    if ($pinnedSessionIds.get().includes(sessionId)) {
-      unpinSession(sessionId)
+    // Pin on the durable lineage-root id so the pin survives auto-compression.
+    const session = $sessions.get().find(s => s.id === sessionId || s._lineage_root_id === sessionId)
+    const pinId = session ? sessionPinId(session) : sessionId
+
+    if ($pinnedSessionIds.get().includes(pinId)) {
+      unpinSession(pinId)
     } else {
-      pinSession(sessionId)
+      pinSession(pinId)
     }
   }, [])