fix(desktop): clear the transcript on every cold resume so sessions can't share one

resumeSession hand-rolls $messages (it paints before a runtime id is bound), and
only cleared the old transcript on the cold path at entry. But a warm-cache hit
can bail down to the full resume — an empty-transcript drop, or the cache being
purged during the profile-swap await — without ever clearing, so the previous
session's array leaked into the next one. Symptom: switching sessions kept
showing the same messages (deterministic once tiling pre-warms the cache on
boot). Clear $messages at the single point every cold/bail path converges, so
carryover is structurally impossible; the warm fast-path still repaints in place.

apps/desktop/src/app/session/hooks/use-session-actions/index.ts

@@ -536,6 +536,16 @@ export function useSessionActions({
       setFreshDraftReady(false)
       setActiveSessionId(null)
       activeSessionIdRef.current = null
+      // A warm-cache hit at entry skipped the cold-path transcript clear, but the
+      // warm path can still bail down to here — an empty-transcript drop, or the
+      // cache getting purged during the profile-swap await — so the PREVIOUS
+      // session's transcript would leak into this cold resume ("switching
+      // sessions shows the same messages"). Clear it so the loader/prefetch
+      // paints fresh; guarded so the normal cold path (already cleared) no-ops.
+      if ($messages.get().length > 0) {
+        setMessages([])
+      }
+
       busyRef.current = true
       setBusy(true)
       setAwaitingResponse(false)