fix: improve markdown export spacing

[singhvishalkr]

Summary

• add a shared Markdown formatter that separates exported messages with blank lines around ---
• reuse the formatter for full-session and selected-message Markdown exports
• avoid rendering empty raw sources values in selected-message Markdown output

Closes #93

Verification

• python -m pytest backend/tests/test_api.py -k export -q
• python -m pytest backend/tests -q
• python -m py_compile backend/routes/export.py backend/tests/test_api.py
• python -m black --check backend/routes/export.py
• git diff --check

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