fix(create): make draft clearing reversible

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Closes #428

What changed

• keep the existing autosaved policy draft restore flow intact after the latest master changes
• make explicit draft clearing reversible through the shared policy draft autosave hook
• prevent the restore path from treating a freshly cleared draft backup as stale just because the saved draft timestamp is older than the clear action
• add a focused page test around discard, undo, and saving the restored draft back to localStorage

Verification

• pnpm exec vitest run src/app/create/create-page-client.test.tsx src/hooks/use-autosave.test.ts
• pnpm exec next lint --file src/app/create/create-page-client.tsx --file src/app/create/create-page-client.test.tsx --file src/hooks/use-autosave.ts
• git diff --check

Note: lint exits cleanly, with the existing react-hooks/exhaustive-deps warning in create-page-client.tsx still reported.