Show import status during uploads

[cbravobernal]

Summary

• Add importer-screen JavaScript that shows an import-started status, spinner, busy state, and prevents duplicate submits.
• Preserve no-JavaScript fallback and existing result summary.
• Distinguish complete, complete-with-warnings, and failure result headlines.
• Add manual verification and pure helper coverage for warnings.

Radical Pipelines

• spec-writer + spec-reviewer approved
• plan-writer + plan-reviewer requested changes
• plan-writer-2 + plan-reviewer-2 approved
• implementer completed changes
• code-reviewer approved

Verification

• PHP lint excluding private/local paths
• php tests/pure-helper-tests.php
• git diff --check

Fixes #9