fix: improve Local Studio import feedback

[hyeokjun32]

Summary

• Changed the Import card default path to the existing results/latest.json fixture.
• Surfaced backend error details in Run, Import, and Jetson helper status messages.
• Added a file:// protocol notice so Studio is opened through the local FastAPI route for API-backed workflows.
• Added tests covering the default import path and static error-handling contract.

Validation

• poetry run python3 -m pytest tests/test_studio_routes.py -q
• poetry run python3 -m pytest -q
• Manual HTTP check for /studio v6 assets and POST /studio/api/import with results/latest.json

Impact

• Makes Import failures actionable instead of showing a generic error.
• Prevents confusion between missing result files and opening the Studio HTML directly from disk.

Notes

• No DB, queue, auth, upload, or production SaaS behavior was added.