Add dependency health checks and warning banner

[saurabhhhcodes]

What changed

• Added a backend /health/dependencies endpoint that reports whether native tools like poppler-utils, tesseract-ocr, and optional ghostscript are present.
• Added a frontend warning banner in the shared tool layout so users see missing-server-dependency guidance before using the converters.
• Documented the manual install commands in the repo README.

Why

When native conversion tools are missing from the server image, PDF and OCR flows can fail in ways that are hard for users to interpret. This makes the dependency gap visible and gives a direct install path.

Validation

• python3 -m py_compile backend/main.py backend/app/__init__.py backend/utils/dependency_checker.py
• git diff --check
• npm run build in frontend/
• Manual browser check of /pdf-to-png with the warning banner rendered above the tool card

Closes #329

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