Automatically migrate old localStorage format where user object has userId field
to new format with id field. This prevents users from needing to re-login after
the backend API response changed from userId to id.
Changes:
- Added migrate function to Zustand persist config to handle userId → id migration
- Added post-hydration safety check in onRehydrateStorage callback
- Added detailed console.log for debugging migration process
Fixes: User data mismatch after API response format change
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Co-Authored-By: Claude <noreply@anthropic.com>
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