Feature: Extend import factory with markitdown-backed formats (Phase 3 of #60)

[EmbarrassingMoment]

Summary

Extend UMarkdownAssetImportFactory so that dropping a PDF / DOCX / PPTX / HTML file into the Content Browser produces a UMarkdownAsset whose body is the markitdown-converted output.

Scope

• Register additional Formats entries on the factory (initial set: pdf, docx, pptx, html)
• Route those formats through the converter from the Phase 2 issue
• Persist the original source path so Reimport re-runs markitdown
• Localized error message when the executable is missing or conversion fails

Out of scope

• OCR / audio inputs (require additional markitdown extras; revisit later)
• Batch UI (separate issue)

Dependencies

• Blocked on the Phase 2 converter issue
• Parent: Feature: Add "Convert to Markdown" Editor Utility Widget using markitdown (Python) #60