Showcase v2 typed spans in the Streamlit demo

[adaamko]

Idea

demo/streamlit_demo.py currently shows v1 binary spans. Update it to showcase the v2 detectors and their typed output: each flagged span with its hallucination category (and subcategory), color-coded.

Done when

• The demo can load a v2 model — generative KRLabsOrg/lettucedect-v2-qwen-2b, or encoder KRLabsOrg/lettucedect-v2-mmbert-base + taxonomy_head="KRLabsOrg/lettucedect-v2-taxonomy-head".
• Spans render with category/subcategory labels.

Good first issue — UI wiring over the existing HallucinationDetector API.