The vault path was stored only in React state (useState), which resets
on every app restart. Now the last active vault path is written to
~/Library/Application Support/com.laputa.app/last-vault.txt on every
vault switch, and loaded on startup. If the saved vault no longer exists,
the existing onboarding flow shows the vault picker instead of silently
falling back to the demo vault.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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