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/store/session.ts

@@ -16,6 +16,12 @@ function updateAtom<T>(store: AppAtom<T>, next: Updater<T>) {
   store.set(typeof next === 'function' ? (next as (current: T) => T)(store.get()) : next)
 }
 
+/** Durable id for pinning. Auto-compression rotates a conversation's session
+ *  id (root -> continuation tip), so pins keyed on the live id evaporate. The
+ *  lineage root is stable across every compression, so we pin on that. */
+export const sessionPinId = (session: Pick<SessionInfo, '_lineage_root_id' | 'id'>): string =>
+  session._lineage_root_id ?? session.id
+
 export const $connection = atom<HermesConnection | null>(null)
 export const $gatewayState = atom('idle')
 export const $sessions = atom<SessionInfo[]>([])