fix: Fix toast identity race handling

[the-asad-iqbal]

This PR fixes two high-severity toast identity issues:

• Unnamed toasts now get unique generated ids instead of sharing a constant default id.
• Dismiss now targets the exact (id, instanceId) pair, so delayed removal cannot delete a newer toast with the same id.

Changes

• Updated toast creation to use a unique id when id is not provided.
• Updated dismiss flow to:
  • capture the current instanceId
  • mark exiting only for that specific instance
  • remove only that specific instance after exit timeout

Result

• Multiple unnamed toasts no longer overwrite each other.
• Recreated toasts with the same id are safe from stale dismiss timers.