Fix Scratch import/export reliability in IDE

[TopProjectsCreator]

Motivation

• Prevent assets from being dropped during Scratch export when fileNames is stale or incomplete.
• Make the Scratch import/export UI more robust so users can re-import the same file and downloads behave reliably in browsers.

Description

• Update exportSb3 in src/services/scratchSb3.ts to export the union of archive.fileNames and Object.keys(archive.files) so assets present in files are always included.
• Harden handleImport in src/components/scratch/ScratchPanel.tsx by resetting the hidden file input value after each import attempt (importInputRef.current.value = '') so the same file can be re-imported.
• Wrap handleExport in src/components/scratch/ScratchPanel.tsx with try/catch, use a safer browser download flow (document.body.appendChild(a) → a.click() → a.remove() → delayed URL.revokeObjectURL), and surface failures via setVmError.
• Add a regression test src/test/scratchSb3.test.ts that verifies assets in files are exported even when fileNames only contains project.json.

Testing

• Ran npm test -- --run src/test/scratchSb3.test.ts, which completed successfully with 2 tests passed.
• Existing round-trip import/export test still passes after the change.

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Codex Task